Network Working Group Jon Callas
Category: INTERNET-DRAFT PGP Corporation
draft-ietf-openpgp-rfc2440bis-17.txt
Expires November 2006 Lutz Donnerhacke
May 2006
Obsoletes: 1991, 2440 Hal Finney
PGP Corporation
David Shaw
Rodney Thayer
OpenPGP Message Format
draft-ietf-openpgp-rfc2440bis-17.txt
Copyright (C) The Internet Society (2006).
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Internet-Drafts are draft documents valid for a maximum of six
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The list of current Internet-Drafts can be accessed at
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IANA Considerations
This document defines many tag values, yet it doesn't describe a
mechanism for adding new tags (for new features). Traditionally the
Internet Assigned Numbers Authority (IANA) handles the allocation of
new values for future expansion and RFCs usually define the
procedure to be used by the IANA. However there are subtle (and not
so subtle) interactions that may occur in this protocol between new
features and existing features which result in a significant
reduction in over all security. Therefore this document does not
define an extension procedure. Instead requests to define new tag
values (say for new encryption algorithms for example) should be
forwarded to the IESG Security Area Directors for consideration or
forwarding to the appropriate IETF Working Group for consideration.
Abstract
This document is maintained in order to publish all necessary
information needed to develop interoperable applications based on
the OpenPGP format. It is not a step-by-step cookbook for writing an
application. It describes only the format and methods needed to
read, check, generate, and write conforming packets crossing any
network. It does not deal with storage and implementation questions.
It does, however, discuss implementation issues necessary to avoid
security flaws.
OpenPGP software uses a combination of strong public-key and
symmetric cryptography to provide security services for electronic
communications and data storage. These services include
confidentiality, key management, authentication, and digital
signatures. This document specifies the message formats used in
OpenPGP.
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This document provides information on the message-exchange packet
formats used by OpenPGP to provide encryption, decryption, signing,
and key management functions. It is a revision of RFC 2440, "OpenPGP
Message Format", which itself replaces RFC 1991, "PGP Message
Exchange Formats."
1.1. Terms
* OpenPGP - This is a definition for security software that uses
PGP 5.x as a basis, formalized in RFC 2440 and this document.
* PGP - Pretty Good Privacy. PGP is a family of software systems
developed by Philip R. Zimmermann from which OpenPGP is based.
* PGP 2.6.x - This version of PGP has many variants, hence the
term PGP 2.6.x. It used only RSA, MD5, and IDEA for its
cryptographic transforms. An informational RFC, RFC 1991, was
written describing this version of PGP.
* PGP 5.x - This version of PGP is formerly known as "PGP 3" in
the community and also in the predecessor of this document, RFC1991. It has new formats and corrects a number of problems in
the PGP 2.6.x design. It is referred to here as PGP 5.x because
that software was the first release of the "PGP 3" code base.
* GPG - GNU Privacy Guard, also called GnuPG. GPG is an OpenPGP
implementation that avoids all encumbered algorithms.
Consequently, early versions of GPG did not include RSA public
keys. GPG may or may not have (depending on version) support for
IDEA or other encumbered algorithms.
"PGP", "Pretty Good", and "Pretty Good Privacy" are trademarks of
PGP Corporation and are used with permission.
This document uses the terms "MUST", "SHOULD", and "MAY" as defined
in RFC 2119, along with the negated forms of those terms.
2. General functions
OpenPGP provides data integrity services for messages and data files
by using these core technologies:
- digital signatures
- encryption
- compression
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- radix-64 conversion
In addition, OpenPGP provides key management and certificate
services, but many of these are beyond the scope of this document.
2.1. Confidentiality via Encryption
OpenPGP combines symmetric-key encryption and public key encryption
to provide confidentiality. When made confidential, first the object
is encrypted using a symmetric encryption algorithm. Each symmetric
key is used only once, for a single object. A new "session key" is
generated as a random number for each object (sometimes referred to
as a session). Since it is used only once, the session key is bound
to the message and transmitted with it. To protect the key, it is
encrypted with the receiver's public key. The sequence is as
follows:
1. The sender creates a message.
2. The sending OpenPGP generates a random number to be used as a
session key for this message only.
3. The session key is encrypted using each recipient's public key.
These "encrypted session keys" start the message.
4. The sending OpenPGP encrypts the message using the session key,
which forms the remainder of the message. Note that the message
is also usually compressed.
5. The receiving OpenPGP decrypts the session key using the
recipient's private key.
6. The receiving OpenPGP decrypts the message using the session
key. If the message was compressed, it will be decompressed.
With symmetric-key encryption, an object may be encrypted with a
symmetric key derived from a passphrase (or other shared secret), or
a two-stage mechanism similar to the public-key method described
above in which a session key is itself encrypted with a symmetric
algorithm keyed from a shared secret.
Both digital signature and confidentiality services may be applied
to the same message. First, a signature is generated for the message
and attached to the message. Then, the message plus signature is
encrypted using a symmetric session key. Finally, the session key is
encrypted using public-key encryption and prefixed to the encrypted
block.
2.2. Authentication via Digital signature
The digital signature uses a hash code or message digest algorithm,
and a public-key signature algorithm. The sequence is as follows:
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1. The sender creates a message.
2. The sending software generates a hash code of the message.
3. The sending software generates a signature from the hash code
using the sender's private key.
4. The binary signature is attached to the message.
5. The receiving software keeps a copy of the message signature.
6. The receiving software generates a new hash code for the
received message and verifies it using the message's signature.
If the verification is successful, the message is accepted as
authentic.
2.3. Compression
OpenPGP implementations SHOULD compress the message after applying
the signature but before encryption.
If an implementation does not implement compression, its authors
should be aware that most OpenPGP messages in the world are
compressed. Thus, it may even be wise for a space-constrained
implementation to implement decompression, but not compression.
Furthermore, compression has the added side-effect that some types
of attacks can be thwarted by the fact that slightly altered,
compressed data rarely uncompresses without severe errors. This is
hardly rigorous, but it is operationally useful. These attacks can
be rigorously prevented by implementing and using Modification
Detection Codes as described in sections following.
2.4. Conversion to Radix-64
OpenPGP's underlying native representation for encrypted messages,
signature certificates, and keys is a stream of arbitrary octets.
Some systems only permit the use of blocks consisting of seven-bit,
printable text. For transporting OpenPGP's native raw binary octets
through channels that are not safe to raw binary data, a printable
encoding of these binary octets is needed. OpenPGP provides the
service of converting the raw 8-bit binary octet stream to a stream
of printable ASCII characters, called Radix-64 encoding or ASCII
Armor.
Implementations SHOULD provide Radix-64 conversions.
2.5. Signature-Only Applications
OpenPGP is designed for applications that use both encryption and
signatures, but there are a number of problems that are solved by a
signature-only implementation. Although this specification requires
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both encryption and signatures, it is reasonable for there to be
subset implementations that are non-conformant only in that they
omit encryption.
3. Data Element Formats
This section describes the data elements used by OpenPGP.
3.1. Scalar numbers
Scalar numbers are unsigned, and are always stored in big-endian
format. Using n[k] to refer to the kth octet being interpreted, the
value of a two-octet scalar is ((n[0] << 8) + n[1]). The value of a
four-octet scalar is ((n[0] << 24) + (n[1] << 16) + (n[2] << 8) +
n[3]).
3.2. Multiprecision Integers
Multiprecision Integers (also called MPIs) are unsigned integers
used to hold large integers such as the ones used in cryptographic
calculations.
An MPI consists of two pieces: a two-octet scalar that is the length
of the MPI in bits followed by a string of octets that contain the
actual integer.
These octets form a big-endian number; a big-endian number can be
made into an MPI by prefixing it with the appropriate length.
Examples:
(all numbers are in hexadecimal)
The string of octets [00 01 01] forms an MPI with the value 1. The
string [00 09 01 FF] forms an MPI with the value of 511.
Additional rules:
The size of an MPI is ((MPI.length + 7) / 8) + 2 octets.
The length field of an MPI describes the length starting from its
most significant non-zero bit. Thus, the MPI [00 02 01] is not
formed correctly. It should be [00 01 01].
Unused bits of an MPI MUST be zero.
Also note that when an MPI is encrypted, the length refers to the
plaintext MPI. It may be ill-formed in its ciphertext.
3.3. Key IDs
A Key ID is an eight-octet scalar that identifies a key.
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Implementations SHOULD NOT assume that Key IDs are unique. The
section, "Enhanced Key Formats" below describes how Key IDs are
formed.
3.4. Text
Unless otherwise specified, the character set for text is the UTF-8
[RFC2279] encoding of Unicode [ISO10646].
3.5. Time fields
A time field is an unsigned four-octet number containing the number
of seconds elapsed since midnight, 1 January 1970 UTC.
3.6. Keyrings
A keyring is a collection of one or more keys in a file or database.
Traditionally, a keyring is simply a sequential list of keys, but
may be any suitable database. It is beyond the scope of this
standard to discuss the details of keyrings or other databases.
3.7. String-to-key (S2K) specifiers
String-to-key (S2K) specifiers are used to convert passphrase
strings into symmetric-key encryption/decryption keys. They are used
in two places, currently: to encrypt the secret part of private keys
in the private keyring, and to convert passphrases to encryption
keys for symmetrically encrypted messages.
3.7.1. String-to-key (S2K) specifier types
There are three types of S2K specifiers currently supported, and
some reserved values:
ID S2K Type
-- --- ----
0 Simple S2K
1 Salted S2K
2 Reserved value
3 Iterated and Salted S2K
100 to 110 Private/Experimental S2K
These are described as follows:
3.7.1.1. Simple S2K
This directly hashes the string to produce the key data. See below
for how this hashing is done.
Octet 0: 0x00
Octet 1: hash algorithm
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Simple S2K hashes the passphrase to produce the session key. The
manner in which this is done depends on the size of the session key
(which will depend on the cipher used) and the size of the hash
algorithm's output. If the hash size is greater than the session key
size, the high-order (leftmost) octets of the hash are used as the
key.
If the hash size is less than the key size, multiple instances of
the hash context are created -- enough to produce the required key
data. These instances are preloaded with 0, 1, 2, ... octets of
zeros (that is to say, the first instance has no preloading, the
second gets preloaded with 1 octet of zero, the third is preloaded
with two octets of zeros, and so forth).
As the data is hashed, it is given independently to each hash
context. Since the contexts have been initialized differently, they
will each produce different hash output. Once the passphrase is
hashed, the output data from the multiple hashes is concatenated,
first hash leftmost, to produce the key data, with any excess octets
on the right discarded.
3.7.1.2. Salted S2K
This includes a "salt" value in the S2K specifier -- some arbitrary
data -- that gets hashed along with the passphrase string, to help
prevent dictionary attacks.
Octet 0: 0x01
Octet 1: hash algorithm
Octets 2-9: 8-octet salt value
Salted S2K is exactly like Simple S2K, except that the input to the
hash function(s) consists of the 8 octets of salt from the S2K
specifier, followed by the passphrase.
3.7.1.3. Iterated and Salted S2K
This includes both a salt and an octet count. The salt is combined
with the passphrase and the resulting value is hashed repeatedly.
This further increases the amount of work an attacker must do to try
dictionary attacks.
Octet 0: 0x03
Octet 1: hash algorithm
Octets 2-9: 8-octet salt value
Octet 10: count, a one-octet, coded value
The count is coded into a one-octet number using the following
formula:
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#define EXPBIAS 6
count = ((Int32)16 + (c & 15)) << ((c >> 4) + EXPBIAS);
The above formula is in C, where "Int32" is a type for a 32-bit
integer, and the variable "c" is the coded count, Octet 10.
Iterated-Salted S2K hashes the passphrase and salt data multiple
times. The total number of octets to be hashed is specified in the
encoded count in the S2K specifier. Note that the resulting count
value is an octet count of how many octets will be hashed, not an
iteration count.
Initially, one or more hash contexts are set up as with the other
S2K algorithms, depending on how many octets of key data are needed.
Then the salt, followed by the passphrase data is repeatedly hashed
until the number of octets specified by the octet count has been
hashed. The one exception is that if the octet count is less than
the size of the salt plus passphrase, the full salt plus passphrase
will be hashed even though that is greater than the octet count.
After the hashing is done the data is unloaded from the hash
context(s) as with the other S2K algorithms.
3.7.2. String-to-key usage
Implementations SHOULD use salted or iterated-and-salted S2K
specifiers, as simple S2K specifiers are more vulnerable to
dictionary attacks.
3.7.2.1. Secret key encryption
An S2K specifier can be stored in the secret keyring to specify how
to convert the passphrase to a key that unlocks the secret data.
Older versions of PGP just stored a cipher algorithm octet preceding
the secret data or a zero to indicate that the secret data was
unencrypted. The MD5 hash function was always used to convert the
passphrase to a key for the specified cipher algorithm.
For compatibility, when an S2K specifier is used, the special value
254 or 255 is stored in the position where the hash algorithm octet
would have been in the old data structure. This is then followed
immediately by a one-octet algorithm identifier, and then by the S2K
specifier as encoded above.
Therefore, preceding the secret data there will be one of these
possibilities:
0: secret data is unencrypted (no passphrase)
255 or 254: followed by algorithm octet and S2K specifier
Cipher alg: use Simple S2K algorithm using MD5 hash
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This last possibility, the cipher algorithm number with an implicit
use of MD5 and IDEA, is provided for backward compatibility; it MAY
be understood, but SHOULD NOT be generated, and is deprecated.
These are followed by an Initial Vector of the same length as the
block size of the cipher for the decryption of the secret values, if
they are encrypted, and then the secret key values themselves.
3.7.2.2. Symmetric-key message encryption
OpenPGP can create a Symmetric-key Encrypted Session Key (ESK)
packet at the front of a message. This is used to allow S2K
specifiers to be used for the passphrase conversion or to create
messages with a mix of symmetric-key ESKs and public-key ESKs. This
allows a message to be decrypted either with a passphrase or a
public key pair.
PGP 2.X always used IDEA with Simple string-to-key conversion when
encrypting a message with a symmetric algorithm. This is deprecated,
but MAY be used for backward-compatibility.
4. Packet Syntax
This section describes the packets used by OpenPGP.
4.1. Overview
An OpenPGP message is constructed from a number of records that are
traditionally called packets. A packet is a chunk of data that has a
tag specifying its meaning. An OpenPGP message, keyring,
certificate, and so forth consists of a number of packets. Some of
those packets may contain other OpenPGP packets (for example, a
compressed data packet, when uncompressed, contains OpenPGP
packets).
Each packet consists of a packet header, followed by the packet
body. The packet header is of variable length.
4.2. Packet Headers
The first octet of the packet header is called the "Packet Tag." It
determines the format of the header and denotes the packet contents.
The remainder of the packet header is the length of the packet.
Note that the most significant bit is the left-most bit, called bit
7. A mask for this bit is 0x80 in hexadecimal.
+---------------+
PTag |7 6 5 4 3 2 1 0|
+---------------+
Bit 7 -- Always one
Bit 6 -- New packet format if set
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PGP 2.6.x only uses old format packets. Thus, software that
interoperates with those versions of PGP must only use old format
packets. If interoperability is not an issue, the new packet format
is preferred. Note that old format packets have four bits of packet
tags, and new format packets have six; some features cannot be used
and still be backward-compatible.
Also note that packets with a tag greater than or equal to 16 MUST
use new format packets. The old format packets can only express tags
less than or equal to 15.
Old format packets contain:
Bits 5-2 -- packet tag
Bits 1-0 - length-type
New format packets contain:
Bits 5-0 -- packet tag
4.2.1. Old-Format Packet Lengths
The meaning of the length-type in old-format packets is:
0 - The packet has a one-octet length. The header is 2 octets long.
1 - The packet has a two-octet length. The header is 3 octets long.
2 - The packet has a four-octet length. The header is 5 octets long.
3 - The packet is of indeterminate length. The header is 1 octet
long, and the implementation must determine how long the packet
is. If the packet is in a file, this means that the packet
extends until the end of the file. In general, an implementation
SHOULD NOT use indeterminate length packets except where the end
of the data will be clear from the context, and even then it is
better to use a definite length, or a new-format header. The
new-format headers described below have a mechanism for
precisely encoding data of indeterminate length.
4.2.2. New-Format Packet Lengths
New format packets have four possible ways of encoding length:
1. A one-octet Body Length header encodes packet lengths of up to
191 octets.
2. A two-octet Body Length header encodes packet lengths of 192 to
8383 octets.
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3. A five-octet Body Length header encodes packet lengths of up to
4,294,967,295 (0xFFFFFFFF) octets in length. (This actually
encodes a four-octet scalar number.)
4. When the length of the packet body is not known in advance by
the issuer, Partial Body Length headers encode a packet of
indeterminate length, effectively making it a stream.
4.2.2.1. One-Octet Lengths
A one-octet Body Length header encodes a length of from 0 to 191
octets. This type of length header is recognized because the one
octet value is less than 192. The body length is equal to:
bodyLen = 1st_octet;
4.2.2.2. Two-Octet Lengths
A two-octet Body Length header encodes a length of from 192 to 8383
octets. It is recognized because its first octet is in the range 192
to 223. The body length is equal to:
bodyLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192
4.2.2.3. Five-Octet Lengths
A five-octet Body Length header consists of a single octet holding
the value 255, followed by a four-octet scalar. The body length is
equal to:
bodyLen = (2nd_octet << 24) | (3rd_octet << 16) |
(4th_octet << 8) | 5th_octet
This basic set of one, two, and five-octet lengths is also used
internally to some packets.
4.2.2.4. Partial Body Lengths
A Partial Body Length header is one octet long and encodes the
length of only part of the data packet. This length is a power of 2,
from 1 to 1,073,741,824 (2 to the 30th power). It is recognized by
its one octet value that is greater than or equal to 224, and less
than 255. The partial body length is equal to:
partialBodyLen = 1 << (1st_octet & 0x1f);
Each Partial Body Length header is followed by a portion of the
packet body data. The Partial Body Length header specifies this
portion's length. Another length header (one octet, two-octet,
five-octet, or partial) follows that portion. The last length header
in the packet MUST NOT be a partial Body Length header. Partial Body
Length headers may only be used for the non-final parts of the
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packet.
Note also that the last Body Length header can be a zero-length
header.
An implementation MAY use Partial Body Lengths for data packets, be
they literal, compressed, or encrypted. The first partial length
MUST be at least 512 octets long. Partial Body Lengths MUST NOT be
used for any other packet types.
4.2.3. Packet Length Examples
These examples show ways that new-format packets might encode the
packet lengths.
A packet with length 100 may have its length encoded in one octet:
0x64. This is followed by 100 octets of data.
A packet with length 1723 may have its length coded in two octets:
0xC5, 0xFB. This header is followed by the 1723 octets of data.
A packet with length 100000 may have its length encoded in five
octets: 0xFF, 0x00, 0x01, 0x86, 0xA0.
It might also be encoded in the following octet stream: 0xEF, first
32768 octets of data; 0xE1, next two octets of data; 0xE0, next one
octet of data; 0xF0, next 65536 octets of data; 0xC5, 0xDD, last
1693 octets of data. This is just one possible encoding, and many
variations are possible on the size of the Partial Body Length
headers, as long as a regular Body Length header encodes the last
portion of the data.
Please note that in all of these explanations, the total length of
the packet is the length of the header(s) plus the length of the
body.
4.3. Packet Tags
The packet tag denotes what type of packet the body holds. Note that
old format headers can only have tags less than 16, whereas new
format headers can have tags as great as 63. The defined tags (in
decimal) are:
0 -- Reserved - a packet tag must not have this value
1 -- Public-Key Encrypted Session Key Packet
2 -- Signature Packet
3 -- Symmetric-Key Encrypted Session Key Packet
4 -- One-Pass Signature Packet
5 -- Secret Key Packet
6 -- Public Key Packet
7 -- Secret Subkey Packet
8 -- Compressed Data Packet
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9 -- Symmetrically Encrypted Data Packet
10 -- Marker Packet
11 -- Literal Data Packet
12 -- Trust Packet
13 -- User ID Packet
14 -- Public Subkey Packet
17 -- User Attribute Packet
18 -- Sym. Encrypted and Integrity Protected Data Packet
19 -- Modification Detection Code Packet
60 to 63 -- Private or Experimental Values
5. Packet Types5.1. Public-Key Encrypted Session Key Packets (Tag 1)
A Public-Key Encrypted Session Key packet holds the session key used
to encrypt a message. Zero or more Public-Key Encrypted Session Key
packets and/or Symmetric-Key Encrypted Session Key packets may
precede a Symmetrically Encrypted Data Packet, which holds an
encrypted message. The message is encrypted with the session key,
and the session key is itself encrypted and stored in the Encrypted
Session Key packet(s). The Symmetrically Encrypted Data Packet is
preceded by one Public-Key Encrypted Session Key packet for each
OpenPGP key to which the message is encrypted. The recipient of the
message finds a session key that is encrypted to their public key,
decrypts the session key, and then uses the session key to decrypt
the message.
The body of this packet consists of:
- A one-octet number giving the version number of the packet type.
The currently defined value for packet version is 3.
- An eight-octet number that gives the key ID of the public key
that the session key is encrypted to. If the session key is
encrypted to a subkey then the key ID of this subkey is used
here instead of the key ID of the primary key.
- A one-octet number giving the public key algorithm used.
- A string of octets that is the encrypted session key. This
string takes up the remainder of the packet, and its contents
are dependent on the public key algorithm used.
Algorithm Specific Fields for RSA encryption
- multiprecision integer (MPI) of RSA encrypted value m**e mod n.
Algorithm Specific Fields for Elgamal encryption:
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- MPI of Elgamal (Diffie-Hellman) value g**k mod p.
- MPI of Elgamal (Diffie-Hellman) value m * y**k mod p.
The value "m" in the above formulas is derived from the session key
as follows. First the session key is prefixed with a one-octet
algorithm identifier that specifies the symmetric encryption
algorithm used to encrypt the following Symmetrically Encrypted Data
Packet. Then a two-octet checksum is appended which is equal to the
sum of the preceding session key octets, not including the algorithm
identifier, modulo 65536. This value is then encoded as described in
PKCS#1 block encoding EME-PKCS1-v1_5 in Section 12.1 to form the "m"
value used in the formulas above.
Note that when an implementation forms several PKESKs with one
session key, forming a message that can be decrypted by several
keys, the implementation MUST make a new PKCS#1 encoding for each
key.
An implementation MAY accept or use a Key ID of zero as a "wild
card" or "speculative" Key ID. In this case, the receiving
implementation would try all available private keys, checking for a
valid decrypted session key. This format helps reduce traffic
analysis of messages.
5.2. Signature Packet (Tag 2)
A signature packet describes a binding between some public key and
some data. The most common signatures are a signature of a file or a
block of text, and a signature that is a certification of a User ID.
Two versions of signature packets are defined. Version 3 provides
basic signature information, while version 4 provides an expandable
format with subpackets that can specify more information about the
signature. PGP 2.6.x only accepts version 3 signatures.
Implementations SHOULD accept V3 signatures. Implementations SHOULD
generate V4 signatures.
Note that if an implementation is creating an encrypted and signed
message that is encrypted to a V3 key, it is reasonable to create a
V3 signature.
5.2.1. Signature Types
There are a number of possible meanings for a signature, which are
indicated in a signature type octet in any given signature. Please
note that the vagueness of these meanings is not a flaw, but a
feature of the system. Because OpenPGP places final authority for
validity upon the receiver of a signature, it may be that one
signer's casual act might be more rigorous than some other
authority's positive act. See section 5.2.4, "Computing
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Signatures," for detailed information on how to compute and verify
signatures of each type.
These meanings are:
0x00: Signature of a binary document.
This means the signer owns it, created it, or certifies that it
has not been modified.
0x01: Signature of a canonical text document.
This means the signer owns it, created it, or certifies that it
has not been modified. The signature is calculated over the text
data with its line endings converted to <CR><LF>.
0x02: Standalone signature.
This signature is a signature of only its own subpacket
contents. It is calculated identically to a signature over a
zero-length binary document. Note that it doesn't make sense to
have a V3 standalone signature.
0x10: Generic certification of a User ID and Public Key packet.
The issuer of this certification does not make any particular
assertion as to how well the certifier has checked that the
owner of the key is in fact the person described by the User ID.
0x11: Persona certification of a User ID and Public Key packet.
The issuer of this certification has not done any verification
of the claim that the owner of this key is the User ID
specified.
0x12: Casual certification of a User ID and Public Key packet.
The issuer of this certification has done some casual
verification of the claim of identity.
0x13: Positive certification of a User ID and Public Key packet.
The issuer of this certification has done substantial
verification of the claim of identity.
Most OpenPGP implementations make their "key signatures" as 0x10
certifications. Some implementations can issue 0x11-0x13
certifications, but few differentiate between the types.
0x18: Subkey Binding Signature
This signature is a statement by the top-level signing key that
indicates that it owns the subkey. This signature is calculated
directly on the primary key and subkey, and not on any User ID
or other packets. A signature that binds a signing subkey MUST
have an embedded signature subpacket in this binding signature
which contains a 0x19 signature made by the signing subkey on
the primary key and subkey.
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0x19 Primary Key Binding Signature
This signature is a statement by a signing subkey, indicating
that it is owned by the primary key and subkey. This signature
is calculated the same way as a 0x18 signature: directly on the
primary key and subkey, and not on any User ID or other packets.
0x1F: Signature directly on a key
This signature is calculated directly on a key. It binds the
information in the signature subpackets to the key, and is
appropriate to be used for subpackets that provide information
about the key, such as the revocation key subpacket. It is also
appropriate for statements that non-self certifiers want to make
about the key itself, rather than the binding between a key and
a name.
0x20: Key revocation signature
The signature is calculated directly on the key being revoked. A
revoked key is not to be used. Only revocation signatures by the
key being revoked, or by an authorized revocation key, should be
considered valid revocation signatures.
0x28: Subkey revocation signature
The signature is calculated directly on the subkey being
revoked. A revoked subkey is not to be used. Only revocation
signatures by the top-level signature key that is bound to this
subkey, or by an authorized revocation key, should be considered
valid revocation signatures.
0x30: Certification revocation signature
This signature revokes an earlier User ID certification
signature (signature class 0x10 through 0x13) or direct-key
signature (0x1F). It should be issued by the same key that
issued the revoked signature or an authorized revocation key.
The signature is computed over the same data as the certificate
that it revokes, and should have a later creation date than that
certificate.
0x40: Timestamp signature.
This signature is only meaningful for the timestamp contained in
it.
0x50: Third-Party Confirmation signature.
This signature is a signature over some other OpenPGP signature
packet(s). It is analogous to a notary seal on the signed data.
A third-party signature SHOULD include Signature Target
subpacket(s) to give easy identification. Note that we really do
mean SHOULD. There are plausible uses for this (such as a blind
party that only sees the signature, not the key nor source
document) that cannot include a target subpacket.
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The body of a version 3 Signature Packet contains:
- One-octet version number (3).
- One-octet length of following hashed material. MUST be 5.
- One-octet signature type.
- Four-octet creation time.
- Eight-octet key ID of signer.
- One-octet public key algorithm.
- One-octet hash algorithm.
- Two-octet field holding left 16 bits of signed hash value.
- One or more multiprecision integers comprising the signature.
This portion is algorithm specific, as described below.
The concatenation of the data to be signed, the signature type and
creation time from the signature packet (5 additional octets) is
hashed. The resulting hash value is used in the signature algorithm.
The high 16 bits (first two octets) of the hash are included in the
signature packet to provide a quick test to reject some invalid
signatures.
Algorithm Specific Fields for RSA signatures:
- multiprecision integer (MPI) of RSA signature value m**d mod n.
Algorithm Specific Fields for DSA signatures:
- MPI of DSA value r.
- MPI of DSA value s.
The signature calculation is based on a hash of the signed data, as
described above. The details of the calculation are different for
DSA signatures than for RSA signatures.
With RSA signatures, the hash value is encoded as described in
PKCS#1 section 9.2.1 encoded using PKCS-1 encoding type
EMSA-PKCS1-v1_5 as described in section 12.1. This requires
inserting the hash value as an octet string into an ASN.1 structure.
The object identifier for the type of hash being used is included in
the structure. The hexadecimal representations for the currently
defined hash algorithms are:
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SHA384: 0x30, 0x41, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02, 0x05,
0x00, 0x04, 0x30
SHA512: 0x30, 0x51, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03, 0x05,
0x00, 0x04, 0x40
DSA signatures MUST use hashes that are equal in size to the number
of bits of q, the group generated by the DSA key's generator value.
If the output size of the chosen hash is larger than the number of
bits of q, the hash result is truncated to fit by taking the number
of leftmost bits equal to the number of bits of q. This (possibly
truncated) hash function result is treated as a number and used
directly in the DSA signature algorithm.
5.2.3. Version 4 Signature Packet Format
The body of a version 4 Signature Packet contains:
- One-octet version number (4).
- One-octet signature type.
- One-octet public key algorithm.
- One-octet hash algorithm.
- Two-octet scalar octet count for following hashed subpacket
data. Note that this is the length in octets of all of the
hashed subpackets; a pointer incremented by this number will
skip over the hashed subpackets.
- Hashed subpacket data set. (zero or more subpackets)
- Two-octet scalar octet count for the following unhashed
subpacket data. Note that this is the length in octets of all of
the unhashed subpackets; a pointer incremented by this number
will skip over the unhashed subpackets.
- Unhashed subpacket data set. (zero or more subpackets)
- Two-octet field holding the left 16 bits of the signed hash
value.
- One or more multiprecision integers comprising the signature.
This portion is algorithm specific, as described above.
The concatenation of the data being signed and the signature data
from the version number through the hashed subpacket data
(inclusive) is hashed. The resulting hash value is what is signed.
The left 16 bits of the hash are included in the signature packet to
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provide a quick test to reject some invalid signatures.
There are two fields consisting of signature subpackets. The first
field is hashed with the rest of the signature data, while the
second is unhashed. The second set of subpackets is not
cryptographically protected by the signature and should include only
advisory information.
The algorithms for converting the hash function result to a
signature are described in a section below.
5.2.3.1. Signature Subpacket Specification
A subpacket data set consists of zero or more signature subpackets.
In signature packets the subpacket data set is preceded by a
two-octet scalar count of the length in octets of all the
subpackets. A pointer incremented by this number will skip over the
subpacket data set.
Each subpacket consists of a subpacket header and a body. The header
consists of:
- the subpacket length (1, 2, or 5 octets)
- the subpacket type (1 octet)
and is followed by the subpacket specific data.
The length includes the type octet but not this length. Its format
is similar to the "new" format packet header lengths, but cannot
have partial body lengths. That is:
if the 1st octet < 192, then
lengthOfLength = 1
subpacketLen = 1st_octet
if the 1st octet >= 192 and < 255, then
lengthOfLength = 2
subpacketLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192
if the 1st octet = 255, then
lengthOfLength = 5
subpacket length = [four-octet scalar starting at 2nd_octet]
The value of the subpacket type octet may be:
2 = signature creation time
3 = signature expiration time
4 = exportable certification
5 = trust signature
6 = regular expression
7 = revocable
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9 = key expiration time
10 = placeholder for backward compatibility
11 = preferred symmetric algorithms
12 = revocation key
16 = issuer key ID
20 = notation data
21 = preferred hash algorithms
22 = preferred compression algorithms
23 = key server preferences
24 = preferred key server
25 = primary User ID
26 = policy URI
27 = key flags
28 = signer's User ID
29 = reason for revocation
30 = features
31 = signature target
32 = embedded signature
100 to 110 = internal or user-defined
An implementation SHOULD ignore any subpacket of a type that it does
not recognize.
Bit 7 of the subpacket type is the "critical" bit. If set, it
denotes that the subpacket is one that is critical for the evaluator
of the signature to recognize. If a subpacket is encountered that is
marked critical but is unknown to the evaluating software, the
evaluator SHOULD consider the signature to be in error.
An evaluator may "recognize" a subpacket, but not implement it. The
purpose of the critical bit is to allow the signer to tell an
evaluator that it would prefer a new, unknown feature to generate an
error than be ignored.
Implementations SHOULD implement "preferences" and the "reason for
revocation" subpackets. Note, however, that if an implementation
chooses not to implement some of the preferences, it is required to
behave in a polite manner to respect the wishes of those users who
do implement these preferences.
5.2.3.2. Signature Subpacket Types
A number of subpackets are currently defined. Some subpackets apply
to the signature itself and some are attributes of the key.
Subpackets that are found on a self-signature are placed on a
certification made by the key itself. Note that a key may have more
than one User ID, and thus may have more than one self-signature,
and differing subpackets.
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A subpacket may be found either in the hashed or unhashed subpacket
sections of a signature. If a subpacket is not hashed, then the
information in it cannot be considered definitive because it is not
part of the signature proper.
5.2.3.3. Notes on Self-Signatures
A self-signature is a binding signature made by the key the
signature refers to. There are three types of self-signatures, the
certification signatures (types 0x10-0x13), the direct-key signature
(type 0x1f), and the subkey binding signature (type 0x18). For
certification self-signatures, each User ID may have a
self-signature, and thus different subpackets in those
self-signatures. For subkey binding signatures, each subkey in fact
has a self-signature. Subpackets that appear in a certification
self-signature apply to the username, and subpackets that appear in
the subkey self-signature apply to the subkey. Lastly, subpackets on
the direct-key signature apply to the entire key.
Implementing software should interpret a self-signature's preference
subpackets as narrowly as possible. For example, suppose a key has
two usernames, Alice and Bob. Suppose that Alice prefers the
symmetric algorithm CAST5, and Bob prefers IDEA or TripleDES. If the
software locates this key via Alice's name, then the preferred
algorithm is CAST5, if software locates the key via Bob's name, then
the preferred algorithm is IDEA. If the key is located by key ID,
the algorithm of the primary User ID of the key provides the
preferred symmetric algorithm.
Revoking a self-signature or allowing it to expire has a semantic
meaning that varies with the signature type. Revoking the
self-signature on a User ID effectively retires that user name. The
self-signature is a statement, "My name X is tied to my signing key
K" and is corroborated by other users' certifications. If another
user revokes their certification, they are effectively saying that
they no longer believe that name and that key are tied together.
Similarly, if the user themselves revokes their self-signature, it
means the user no longer goes by that name, no longer has that email
address, etc. Revoking a binding signature effectively retires that
subkey. Revoking a direct-key signature cancels that signature.
Please see the "Reason for Revocation" subpacket below for more
relevant detail.
Since a self-signature contains important information about the
key's use, an implementation SHOULD allow the user to rewrite the
self-signature, and important information in it, such as preferences
and key expiration.
It is good practice to verify that a self-signature imported into an
implementation doesn't advertise features that the implementation
doesn't support, rewriting the signature as appropriate.
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An implementation that encounters multiple self-signatures on the
same object may resolve the ambiguity in any way it sees fit, but it
is RECOMMENDED that priority be given to the most recent
self-signature.
5.2.3.4. Signature creation time
(4 octet time field)
The time the signature was made.
MUST be present in the hashed area.
5.2.3.5. Issuer
(8 octet key ID)
The OpenPGP key ID of the key issuing the signature.
5.2.3.6. Key expiration time
(4 octet time field)
The validity period of the key. This is the number of seconds after
the key creation time that the key expires. If this is not present
or has a value of zero, the key never expires. This is found only on
a self-signature.
5.2.3.7. Preferred symmetric algorithms
(array of one-octet values)
Symmetric algorithm numbers that indicate which algorithms the key
holder prefers to use. The subpacket body is an ordered list of
octets with the most preferred listed first. It is assumed that only
algorithms listed are supported by the recipient's software.
Algorithm numbers are in section 9. This is only found on a
self-signature.
5.2.3.8. Preferred hash algorithms
(array of one-octet values)
Message digest algorithm numbers that indicate which algorithms the
key holder prefers to receive. Like the preferred symmetric
algorithms, the list is ordered. Algorithm numbers are in section 9.
This is only found on a self-signature.
5.2.3.9. Preferred compression algorithms
(array of one-octet values)
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Compression algorithm numbers that indicate which algorithms the key
holder prefers to use. Like the preferred symmetric algorithms, the
list is ordered. Algorithm numbers are in section 9. If this
subpacket is not included, ZIP is preferred. A zero denotes that
uncompressed data is preferred; the key holder's software might have
no compression software in that implementation. This is only found
on a self-signature.
5.2.3.10. Signature expiration time
(4 octet time field)
The validity period of the signature. This is the number of seconds
after the signature creation time that the signature expires. If
this is not present or has a value of zero, it never expires.
5.2.3.11. Exportable Certification
(1 octet of exportability, 0 for not, 1 for exportable)
This subpacket denotes whether a certification signature is
"exportable," to be used by other users than the signature's issuer.
The packet body contains a Boolean flag indicating whether the
signature is exportable. If this packet is not present, the
certification is exportable; it is equivalent to a flag containing a
1.
Non-exportable, or "local," certifications are signatures made by a
user to mark a key as valid within that user's implementation only.
Thus, when an implementation prepares a user's copy of a key for
transport to another user (this is the process of "exporting" the
key), any local certification signatures are deleted from the key.
The receiver of a transported key "imports" it, and likewise trims
any local certifications. In normal operation, there won't be any,
assuming the import is performed on an exported key. However, there
are instances where this can reasonably happen. For example, if an
implementation allows keys to be imported from a key database in
addition to an exported key, then this situation can arise.
Some implementations do not represent the interest of a single user
(for example, a key server). Such implementations always trim local
certifications from any key they handle.
5.2.3.12. Revocable
(1 octet of revocability, 0 for not, 1 for revocable)
Signature's revocability status. The packet body contains a Boolean
flag indicating whether the signature is revocable. Signatures that
are not revocable have any later revocation signatures ignored. They
represent a commitment by the signer that he cannot revoke his
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signature for the life of his key. If this packet is not present,
the signature is revocable.
5.2.3.13. Trust signature
(1 octet "level" (depth), 1 octet of trust amount)
Signer asserts that the key is not only valid, but also trustworthy,
at the specified level. Level 0 has the same meaning as an ordinary
validity signature. Level 1 means that the signed key is asserted to
be a valid trusted introducer, with the 2nd octet of the body
specifying the degree of trust. Level 2 means that the signed key is
asserted to be trusted to issue level 1 trust signatures, i.e. that
it is a "meta introducer". Generally, a level n trust signature
asserts that a key is trusted to issue level n-1 trust signatures.
The trust amount is in a range from 0-255, interpreted such that
values less than 120 indicate partial trust and values of 120 or
greater indicate complete trust. Implementations SHOULD emit values
of 60 for partial trust and 120 for complete trust.
5.2.3.14. Regular expression
(null-terminated regular expression)
Used in conjunction with trust signature packets (of level > 0) to
limit the scope of trust that is extended. Only signatures by the
target key on User IDs that match the regular expression in the body
of this packet have trust extended by the trust signature subpacket.
The regular expression uses the same syntax as the Henry Spencer's
"almost public domain" regular expression package. A description of
the syntax is found in a section below.
5.2.3.15. Revocation key
(1 octet of class, 1 octet of PK algorithm ID, 20 octets of
fingerprint)
Authorizes the specified key to issue revocation signatures for this
key. Class octet must have bit 0x80 set. If the bit 0x40 is set,
then this means that the revocation information is sensitive. Other
bits are for future expansion to other kinds of authorizations. This
is found on a self-signature.
If the "sensitive" flag is set, the keyholder feels this subpacket
contains private trust information that describes a real-world
sensitive relationship. If this flag is set, implementations SHOULD
NOT export this signature to other users except in cases where the
data needs to be available: when the signature is being sent to the
designated revoker, or when it is accompanied by a revocation
signature from that revoker. Note that it may be appropriate to
isolate this subpacket within a separate signature so that it is not
combined with other subpackets that need to be exported.
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INTERNET-DRAFT OpenPGP Message Format May 11, 20065.2.3.16. Notation Data
(4 octets of flags, 2 octets of name length (M),
2 octets of value length (N),
M octets of name data,
N octets of value data)
This subpacket describes a "notation" on the signature that the
issuer wishes to make. The notation has a name and a value, each of
which are strings of octets. There may be more than one notation in
a signature. Notations can be used for any extension the issuer of
the signature cares to make. The "flags" field holds four octets of
flags.
All undefined flags MUST be zero. Defined flags are:
First octet: 0x80 = human-readable. This note value is text.
Other octets: none.
Notation names are arbitrary strings encoded in UTF-8. They reside
two name spaces: The IETF name space and the user name space.
The IETF name space is registered with IANA. These names MUST NOT
contain the "@" character (0x40). This this is a tag for the user
name space.
Names in the user name space consist of a UTF-8 string tag followed
by "@" followed by a DNS domain name. Note that the tag MUST NOT
contain an "@" character. For example, the "sample" tag used by
Example Corporation could be "sample@example.com".
Names in a user space are owned and controlled by the owners of that
domain. Obviously, it's of bad form to create a new name in a DNS
space that you don't own.
Since the user name space is in the form of an email address,
implementers MAY wish to arrange for that address to reach a person
who can be consulted about the use of the named tag. Note that due
to UTF-8 encoding, not all valid user space name tags are valid
email addresses.
If there is a critical notation, the criticality applies to that
specific notation and not to notations in general.
5.2.3.17. Key server preferences
(N octets of flags)
This is a list of one-bit flags that indicate preferences that the
key holder has about how the key is handled on a key server. All
undefined flags MUST be zero.
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First octet: 0x80 = No-modify
the key holder requests that this key only be modified or
updated by the key holder or an administrator of the key server.
This is found only on a self-signature.
5.2.3.18. Preferred key server
(String)
This is a URI of a key server that the key holder prefers be used
for updates. Note that keys with multiple User IDs can have a
preferred key server for each User ID. Note also that since this is
a URI, the key server can actually be a copy of the key retrieved by
ftp, http, finger, etc.
5.2.3.19. Primary User ID
(1 octet, Boolean)
This is a flag in a User ID's self signature that states whether
this User ID is the main User ID for this key. It is reasonable for
an implementation to resolve ambiguities in preferences, etc. by
referring to the primary User ID. If this flag is absent, its value
is zero. If more than one User ID in a key is marked as primary, the
implementation may resolve the ambiguity in any way it sees fit, but
it is RECOMMENDED that priority be given to the User ID with the
most recent self-signature.
When appearing on a self-signature on a User ID packet, this
subpacket applies only to User ID packets. When appearing on a
self-signature on a User Attribute packet, this subpacket applies
only to User Attribute packets. That is to say, there are two
different and independent "primaries" - one for User IDs, and one
for User Attributes.
5.2.3.20. Policy URI
(String)
This subpacket contains a URI of a document that describes the
policy that the signature was issued under.
5.2.3.21. Key Flags
(N octets of flags)
This subpacket contains a list of binary flags that hold information
about a key. It is a string of octets, and an implementation MUST
NOT assume a fixed size. This is so it can grow over time. If a list
is shorter than an implementation expects, the unstated flags are
considered to be zero. The defined flags are:
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First octet:
0x01 - This key may be used to certify other keys.
0x02 - This key may be used to sign data.
0x04 - This key may be used to encrypt communications.
0x08 - This key may be used to encrypt storage.
0x10 - The private component of this key may have been split by
a secret-sharing mechanism.
0x20 - This key may be used for authentication.
0x80 - The private component of this key may be in the
possession of more than one person.
Usage notes:
The flags in this packet may appear in self-signatures or in
certification signatures. They mean different things depending on
who is making the statement -- for example, a certification
signature that has the "sign data" flag is stating that the
certification is for that use. On the other hand, the
"communications encryption" flag in a self-signature is stating a
preference that a given key be used for communications. Note
however, that it is a thorny issue to determine what is
"communications" and what is "storage." This decision is left wholly
up to the implementation; the authors of this document do not claim
any special wisdom on the issue, and realize that accepted opinion
may change.
The "split key" (0x10) and "group key" (0x80) flags are placed on a
self-signature only; they are meaningless on a certification
signature. They SHOULD be placed only on a direct-key signature
(type 0x1f) or a subkey signature (type 0x18), one that refers to
the key the flag applies to.
5.2.3.22. Signer's User ID
(String)
This subpacket allows a keyholder to state which User ID is
responsible for the signing. Many keyholders use a single key for
different purposes, such as business communications as well as
personal communications. This subpacket allows such a keyholder to
state which of their roles is making a signature.
This subpacket is not appropriate to use to refer to a User
Attribute packet.
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INTERNET-DRAFT OpenPGP Message Format May 11, 20065.2.3.23. Reason for Revocation
(1 octet of revocation code, N octets of reason string)
This subpacket is used only in key revocation and certification
revocation signatures. It describes the reason why the key or
certificate was revoked.
The first octet contains a machine-readable code that denotes the
reason for the revocation:
0x00 - No reason specified (key revocations or cert revocations)
0x01 - Key is superseded (key revocations)
0x02 - Key material has been compromised (key revocations)
0x03 - Key is retired and no longer used (key revocations)
0x20 - User ID information is no longer valid (cert revocations)
Following the revocation code is a string of octets which gives
information about the reason for revocation in human-readable form
(UTF-8). The string may be null, that is, of zero length. The length
of the subpacket is the length of the reason string plus one.
An implementation SHOULD implement this subpacket, include it in all
revocation signatures, and interpret revocations appropriately.
There are important semantic differences between the reasons, and
there are thus important reasons for revoking signatures.
If a key has been revoked because of a compromise, all signatures
created by that key are suspect. However, if it was merely
superseded or retired, old signatures are still valid. If the
revoked signature is the self-signature for certifying a User ID, a
revocation denotes that that user name is no longer in use. Such a
revocation SHOULD include an 0x20 code.
Note that any signature may be revoked, including a certification on
some other person's key. There are many good reasons for revoking a
certification signature, such as the case where the keyholder leaves
the employ of a business with an email address. A revoked
certification is no longer a part of validity calculations.
5.2.3.24. Features
(N octets of flags)
The features subpacket denotes which advanced OpenPGP features a
user's implementation supports. This is so that as features are
added to OpenPGP that cannot be backwards-compatible, a user can
state that they can use that feature. The flags are single bits that
indicate that a given feature is supported.
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This subpacket is similar to a preferences subpacket, and only
appears in a self-signature.
An implementation SHOULD NOT use a feature listed when sending to a
user who does not state that they can use it.
Defined features are:
First octet:
0x01 - Modification Detection (packets 18 and 19)
If an implementation implements any of the defined features, it
SHOULD implement the features subpacket, too.
An implementation may freely infer features from other suitable
implementation-dependent mechanisms.
5.2.3.25. Signature Target
(1 octet PK algorithm, 1 octet hash algorithm, N octets hash)
This subpacket identifies a specific target signature that a
signature refers to. For revocation signatures, this subpacket
provides explicit designation of which signature is being revoked.
For a third-party or timestamp signature, this designates what
signature is signed. All arguments are an identifier of that target
signature.
The N octets of hash data MUST be the size of the hash of the
signature. For example, a target signature with a SHA-1 hash MUST
have 20 octets of hash data.
5.2.3.26. Embedded Signature
(1 signature packet body)
This subpacket contains a complete signature packet body as
specified in section 5.2 above. It is useful when one signature
needs to refer to, or be incorporated in, another signature.
5.2.4. Computing Signatures
All signatures are formed by producing a hash over the signature
data, and then using the resulting hash in the signature algorithm.
For binary document signatures (type 0x00), the document data is
hashed directly. For text document signatures (type 0x01), the
document is canonicalized by converting line endings to <CR><LF>,
and the resulting data is hashed.
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When a signature is made over a key, the hash data starts with the
octet 0x99, followed by a two-octet length of the key, and then body
of the key packet. (Note that this is an old-style packet header for
a key packet with two-octet length.) A subkey binding signature
(type 0x18) or primary key binding signature (type 0x19) then hashes
the subkey using the same format as the main key (also using 0x99 as
the first octet). Key revocation signatures (types 0x20 and 0x28)
hash only the key being revoked.
A certification signature (type 0x10 through 0x13) hashes the User
ID being bound to the key into the hash context after the above
data. A V3 certification hashes the contents of the User ID or
attribute packet packet, without any header. A V4 certification
hashes the constant 0xb4 for User ID certifications or the constant
0xd1 for User Attribute certifications, followed by a four-octet
number giving the length of the User ID or User Attribute data, and
then the User ID or User Attribute data.
When a signature is made over a signature packet (type 0x50), the
hash data starts with the octet 0x88, followed by the four-octet
length of the signature, and then the body of the signature packet.
(Note that this is an old-style packet header for a signature packet
with the length-of-length set to zero). The unhashed subpacket data
of the signature packet being hashed is not included in the hash and
the unhashed subpacket data length value is set to zero.
Once the data body is hashed, then a trailer is hashed. A V3
signature hashes five octets of the packet body, starting from the
signature type field. This data is the signature type, followed by
the four-octet signature time. A V4 signature hashes the packet body
starting from its first field, the version number, through the end
of the hashed subpacket data. Thus, the fields hashed are the
signature version, the signature type, the public key algorithm, the
hash algorithm, the hashed subpacket length, and the hashed
subpacket body.
V4 signatures also hash in a final trailer of six octets: the
version of the signature packet, i.e. 0x04; 0xFF; a four-octet,
big-endian number that is the length of the hashed data from the
signature packet (note that this number does not include these final
six octets.
After all this has been hashed in a single hash context the
resulting hash field is used in the signature algorithm, and placed
at the end of the signature packet.
5.2.4.1. Subpacket Hints
It is certainly possible for a signature to contain conflicting
information in subpackets. For example, a signature may contain
multiple copies of a preference or multiple expiration times. In
most cases, an implementation SHOULD use the last subpacket in the
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signature, but MAY use any conflict resolution scheme that makes
more sense. Please note that we are intentionally leaving conflict
resolution to the implementer; most conflicts are simply syntax
errors, and the wishy-washy language here allows a receiver to be
generous in what they accept, while putting pressure on a creator to
be stingy in what they generate.
Some apparent conflicts may actually make sense -- for example,
suppose a keyholder has an V3 key and a V4 key that share the same
RSA key material. Either of these keys can verify a signature
created by the other, and it may be reasonable for a signature to
contain an issuer subpacket for each key, as a way of explicitly
tying those keys to the signature.
5.3. Symmetric-Key Encrypted Session Key Packets (Tag 3)
The Symmetric-Key Encrypted Session Key packet holds the
symmetric-key encryption of a session key used to encrypt a message.
Zero or more Public-Key Encrypted Session Key packets and/or
Symmetric-Key Encrypted Session Key packets may precede a
Symmetrically Encrypted Data Packet that holds an encrypted message.
The message is encrypted with a session key, and the session key is
itself encrypted and stored in the Encrypted Session Key packet or
the Symmetric-Key Encrypted Session Key packet.
If the Symmetrically Encrypted Data Packet is preceded by one or
more Symmetric-Key Encrypted Session Key packets, each specifies a
passphrase that may be used to decrypt the message. This allows a
message to be encrypted to a number of public keys, and also to one
or more passphrases. This packet type is new, and is not generated
by PGP 2.x or PGP 5.0.
The body of this packet consists of:
- A one-octet version number. The only currently defined version
is 4.
- A one-octet number describing the symmetric algorithm used.
- A string-to-key (S2K) specifier, length as defined above.
- Optionally, the encrypted session key itself, which is decrypted
with the string-to-key object.
If the encrypted session key is not present (which can be detected
on the basis of packet length and S2K specifier size), then the S2K
algorithm applied to the passphrase produces the session key for
decrypting the file, using the symmetric cipher algorithm from the
Symmetric-Key Encrypted Session Key packet.
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If the encrypted session key is present, the result of applying the
S2K algorithm to the passphrase is used to decrypt just that
encrypted session key field, using CFB mode with an IV of all zeros.
The decryption result consists of a one-octet algorithm identifier
that specifies the symmetric-key encryption algorithm used to
encrypt the following Symmetrically Encrypted Data Packet, followed
by the session key octets themselves.
Note: because an all-zero IV is used for this decryption, the S2K
specifier MUST use a salt value, either a Salted S2K or an
Iterated-Salted S2K. The salt value will insure that the decryption
key is not repeated even if the passphrase is reused.
5.4. One-Pass Signature Packets (Tag 4)
The One-Pass Signature packet precedes the signed data and contains
enough information to allow the receiver to begin calculating any
hashes needed to verify the signature. It allows the Signature
Packet to be placed at the end of the message, so that the signer
can compute the entire signed message in one pass.
A One-Pass Signature does not interoperate with PGP 2.6.x or
earlier.
The body of this packet consists of:
- A one-octet version number. The current version is 3.
- A one-octet signature type. Signature types are described in
section 5.2.1.
- A one-octet number describing the hash algorithm used.
- A one-octet number describing the public key algorithm used.
- An eight-octet number holding the key ID of the signing key.
- A one-octet number holding a flag showing whether the signature
is nested. A zero value indicates that the next packet is
another One-Pass Signature packet that describes another
signature to be applied to the same message data.
Note that if a message contains more than one one-pass signature,
then the signature packets bracket the message; that is, the first
signature packet after the message corresponds to the last one-pass
packet and the final signature packet corresponds to the first
one-pass packet.
5.5. Key Material Packet
A key material packet contains all the information about a public or
private key. There are four variants of this packet type, and two
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major versions. Consequently, this section is complex.
5.5.1. Key Packet Variants5.5.1.1. Public Key Packet (Tag 6)
A Public Key packet starts a series of packets that forms an OpenPGP
key (sometimes called an OpenPGP certificate).
5.5.1.2. Public Subkey Packet (Tag 14)
A Public Subkey packet (tag 14) has exactly the same format as a
Public Key packet, but denotes a subkey. One or more subkeys may be
associated with a top-level key. By convention, the top-level key
provides signature services, and the subkeys provide encryption
services.
Note: in PGP 2.6.x, tag 14 was intended to indicate a comment
packet. This tag was selected for reuse because no previous version
of PGP ever emitted comment packets but they did properly ignore
them. Public Subkey packets are ignored by PGP 2.6.x and do not
cause it to fail, providing a limited degree of backward
compatibility.
5.5.1.3. Secret Key Packet (Tag 5)
A Secret Key packet contains all the information that is found in a
Public Key packet, including the public key material, but also
includes the secret key material after all the public key fields.
5.5.1.4. Secret Subkey Packet (Tag 7)
A Secret Subkey packet (tag 7) is the subkey analog of the Secret
Key packet, and has exactly the same format.
5.5.2. Public Key Packet Formats
There are two versions of key-material packets. Version 3 packets
were first generated by PGP 2.6. Version 4 keys first appeared in
PGP 5.0, and are the preferred key version for OpenPGP.
OpenPGP implementations MUST create keys with version 4 format. V3
keys are deprecated; an implementation MUST NOT generate a V3 key,
but MAY accept it.
A version 3 public key or public subkey packet contains:
- A one-octet version number (3).
- A four-octet number denoting the time that the key was created.
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- A two-octet number denoting the time in days that this key is
valid. If this number is zero, then it does not expire.
- A one-octet number denoting the public key algorithm of this key
- A series of multiprecision integers comprising the key material:
- a multiprecision integer (MPI) of RSA public modulus n;
- an MPI of RSA public encryption exponent e.
V3 keys are deprecated. They contain three weaknesses in them.
First, it is relatively easy to construct a V3 key that has the same
key ID as any other key because the key ID is simply the low 64 bits
of the public modulus. Secondly, because the fingerprint of a V3 key
hashes the key material, but not its length, there is an increased
opportunity for fingerprint collisions. Third, there are weaknesses
in the MD5 hash algorithm that make developers prefer other
algorithms. See below for a fuller discussion of key IDs and
fingerprints.
V2 keys are identical to the deprecated V3 keys except for the
version number. An implementation MUST NOT generate them and MAY
accept or reject them as it sees fit.
The version 4 format is similar to the version 3 format except for
the absence of a validity period. This has been moved to the
signature packet. In addition, fingerprints of version 4 keys are
calculated differently from version 3 keys, as described in section
"Enhanced Key Formats."
A version 4 packet contains:
- A one-octet version number (4).
- A four-octet number denoting the time that the key was created.
- A one-octet number denoting the public key algorithm of this key
- A series of multiprecision integers comprising the key material.
This algorithm-specific portion is:
Algorithm Specific Fields for RSA public keys:
- multiprecision integer (MPI) of RSA public modulus n;
- MPI of RSA public encryption exponent e.
Algorithm Specific Fields for DSA public keys:
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- MPI of DSA prime p;
- MPI of DSA group order q (q is a prime divisor of p-1);
- MPI of DSA group generator g;
- MPI of DSA public key value y (= g**x mod p where x is
secret).
Algorithm Specific Fields for Elgamal public keys:
- MPI of Elgamal prime p;
- MPI of Elgamal group generator g;
- MPI of Elgamal public key value y (= g**x mod p where x is
secret).
5.5.3. Secret Key Packet Formats
The Secret Key and Secret Subkey packets contain all the data of the
Public Key and Public Subkey packets, with additional
algorithm-specific secret key data appended, usually in encrypted
form.
The packet contains:
- A Public Key or Public Subkey packet, as described above
- One octet indicating string-to-key usage conventions. Zero
indicates that the secret key data is not encrypted. 255 or 254
indicates that a string-to-key specifier is being given. Any
other value is a symmetric-key encryption algorithm identifier.
- [Optional] If string-to-key usage octet was 255 or 254, a
one-octet symmetric encryption algorithm.
- [Optional] If string-to-key usage octet was 255 or 254, a
string-to-key specifier. The length of the string-to-key
specifier is implied by its type, as described above.
- [Optional] If secret data is encrypted (string-to-key usage
octet not zero), an Initial Vector (IV) of the same length as
the cipher's block size.
- Plain or encrypted multiprecision integers comprising the secret
key data. These algorithm-specific fields are as described
below.
- If the string-to-key usage octet is zero or 255, then a
two-octet checksum of the plaintext of the algorithm-specific
portion (sum of all octets, mod 65536). If the string-to-key
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usage octet was 254, then a 20-octet SHA-1 hash of the plaintext
of the algorithm-specific portion. This checksum or hash is
encrypted together with the algorithm-specific fields (if
string-to-key usage octet is not zero). Note that for all other
values, a two-octet checksum is required.
Algorithm Specific Fields for RSA secret keys:
- multiprecision integer (MPI) of RSA secret exponent d.
- MPI of RSA secret prime value p.
- MPI of RSA secret prime value q (p < q).
- MPI of u, the multiplicative inverse of p, mod q.
Algorithm Specific Fields for DSA secret keys:
- MPI of DSA secret exponent x.
Algorithm Specific Fields for Elgamal secret keys:
- MPI of Elgamal secret exponent x.
Secret MPI values can be encrypted using a passphrase. If a
string-to-key specifier is given, that describes the algorithm for
converting the passphrase to a key, else a simple MD5 hash of the
passphrase is used. Implementations MUST use a string-to-key
specifier; the simple hash is for backward compatibility and is
deprecated, though implementations MAY continue to use existing
private keys in the old format. The cipher for encrypting the MPIs
is specified in the secret key packet.
Encryption/decryption of the secret data is done in CFB mode using
the key created from the passphrase and the Initial Vector from the
packet. A different mode is used with V3 keys (which are only RSA)
than with other key formats. With V3 keys, the MPI bit count prefix
(i.e., the first two octets) is not encrypted. Only the MPI
non-prefix data is encrypted. Furthermore, the CFB state is
resynchronized at the beginning of each new MPI value, so that the
CFB block boundary is aligned with the start of the MPI data.
With V4 keys, a simpler method is used. All secret MPI values are
encrypted in CFB mode, including the MPI bitcount prefix.
The two-octet checksum that follows the algorithm-specific portion
is the algebraic sum, mod 65536, of the plaintext of all the
algorithm-specific octets (including MPI prefix and data). With V3
keys, the checksum is stored in the clear. With V4 keys, the
checksum is encrypted like the algorithm-specific data. This value
is used to check that the passphrase was correct. However, this
checksum is deprecated; an implementation SHOULD NOT use it, but
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should rather use the SHA-1 hash denoted with a usage octet of 254.
The reason for this is that there are some attacks that involve
undetectably modifying the secret key.
5.6. Compressed Data Packet (Tag 8)
The Compressed Data packet contains compressed data. Typically, this
packet is found as the contents of an encrypted packet, or following
a Signature or One-Pass Signature packet, and contains a literal
data packet.
The body of this packet consists of:
- One octet that gives the algorithm used to compress the packet.
- The remainder of the packet is compressed data.
A Compressed Data Packet's body contains an block that compresses
some set of packets. See section "Packet Composition" for details on
how messages are formed.
ZIP-compressed packets are compressed with raw RFC 1951 DEFLATE
blocks. Note that PGP V2.6 uses 13 bits of compression. If an
implementation uses more bits of compression, PGP V2.6 cannot
decompress it.
ZLIB-compressed packets are compressed with RFC 1950 ZLIB-style
blocks.
BZip2-compressed packets are compressed using the BZip2 algorithm.
5.7. Symmetrically Encrypted Data Packet (Tag 9)
The Symmetrically Encrypted Data packet contains data encrypted with
a symmetric-key algorithm. When it has been decrypted, it contains
other packets (usually a literal data packet or compressed data
packet, but in theory other Symmetrically Encrypted Data Packets or
sequences of packets that form whole OpenPGP messages).
The body of this packet consists of:
- Encrypted data, the output of the selected symmetric-key cipher
operating in OpenPGP's variant of Cipher Feedback (CFB) mode.
The symmetric cipher used may be specified in an Public-Key or
Symmetric-Key Encrypted Session Key packet that precedes the
Symmetrically Encrypted Data Packet. In that case, the cipher
algorithm octet is prefixed to the session key before it is
encrypted. If no packets of these types precede the encrypted data,
the IDEA algorithm is used with the session key calculated as the
MD5 hash of the passphrase, though this use is deprecated.
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The data is encrypted in CFB mode, with a CFB shift size equal to
the cipher's block size. The Initial Vector (IV) is specified as all
zeros. Instead of using an IV, OpenPGP prefixes a string of length
equal to the block size of the cipher plus two to the data before it
is encrypted. The first block-size octets (for example, 8 octets for
a 64-bit block length) are random, and the following two octets are
copies of the last two octets of the IV. For example, in an 8 octet
block, octet 9 is a repeat of octet 7, and octet 10 is a repeat of
octet 8. In a cipher of length 16, octet 17 is a repeat of octet 15
and octet 18 is a repeat of octet 16. As a pedantic clarification,
in both these examples, we consider the first octet to be numbered
1.
After encrypting the first block-size-plus-two octets, the CFB state
is resynchronized. The last block-size octets of ciphertext are
passed through the cipher and the block boundary is reset.
The repetition of 16 bits in the random data prefixed to the message
allows the receiver to immediately check whether the session key is
incorrect. See the Security Considerations section for hints on the
proper use of this "quick check."
5.8. Marker Packet (Obsolete Literal Packet) (Tag 10)
An experimental version of PGP used this packet as the Literal
packet, but no released version of PGP generated Literal packets
with this tag. With PGP 5.x, this packet has been re-assigned and is
reserved for use as the Marker packet.
The body of this packet consists of:
- The three octets 0x50, 0x47, 0x50 (which spell "PGP" in UTF-8).
Such a packet MUST be ignored when received. It may be placed at the
beginning of a message that uses features not available in PGP 2.6.x
in order to cause that version to report that newer software is
necessary to process the message.
5.9. Literal Data Packet (Tag 11)
A Literal Data packet contains the body of a message; data that is
not to be further interpreted.
The body of this packet consists of:
- A one-octet field that describes how the data is formatted.
If it is a 'b' (0x62), then the literal packet contains binary data.
If it is a 't' (0x74), then it contains text data, and thus may need
line ends converted to local form, or other text-mode changes. The
tag 'u' (0x75) means the same as 't', but also indicates that
implementation believes that the literal data contains UTF-8 text.
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Early versions of PGP also defined a value of 'l' as a 'local' mode
for machine-local conversions. RFC 1991 incorrectly stated this
local mode flag as '1' (ASCII numeral one). Both of these local
modes are deprecated.
- File name as a string (one-octet length, followed by a file
name). This may be a zero-length string. Commonly, if the source
of the encrypted data is a file, this will be the name of the
encrypted file. An implementation MAY consider the file name in
the literal packet to be a more authoritative name than the
actual file name.
If the special name "_CONSOLE" is used, the message is considered to
be "for your eyes only". This advises that the message data is
unusually sensitive, and the receiving program should process it
more carefully, perhaps avoiding storing the received data to disk,
for example.
- A four-octet number that indicates a date associated with the
literal data. Commonly, the date might be the modification date
of a file, or the time the packet was created, or a zero that
indicates no specific time.
- The remainder of the packet is literal data.
Text data is stored with <CR><LF> text endings (i.e. network-normal
line endings). These should be converted to native line endings by
the receiving software.
5.10. Trust Packet (Tag 12)
The Trust packet is used only within keyrings and is not normally
exported. Trust packets contain data that record the user's
specifications of which key holders are trustworthy introducers,
along with other information that implementing software uses for
trust information. The format of trust packets is defined by a given
implementation.
Trust packets SHOULD NOT be emitted to output streams that are
transferred to other users, and they SHOULD be ignored on any input
other than local keyring files.
5.11. User ID Packet (Tag 13)
A User ID packet consists of UTF-8 text that is intended to
represent the name and email address of the key holder. By
convention, it includes an RFC 2822 mail name-addr, but there are no
restrictions on its content. The packet length in the header
specifies the length of the User ID.
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The User Attribute packet is a variation of the User ID packet. It
is capable of storing more types of data than the User ID packet
which is limited to text. Like the User ID packet, a User Attribute
packet may be certified by the key owner ("self-signed") or any
other key owner who cares to certify it. Except as noted, a User
Attribute packet may be used anywhere that a User ID packet may be
used.
While User Attribute packets are not a required part of the OpenPGP
standard, implementations SHOULD provide at least enough
compatibility to properly handle a certification signature on the
User Attribute packet. A simple way to do this is by treating the
User Attribute packet as a User ID packet with opaque contents, but
an implementation may use any method desired.
The User Attribute packet is made up of one or more attribute
subpackets. Each subpacket consists of a subpacket header and a
body. The header consists of:
- the subpacket length (1, 2, or 5 octets)
- the subpacket type (1 octet)
and is followed by the subpacket specific data.
The only currently defined subpacket type is 1, signifying an image.
An implementation SHOULD ignore any subpacket of a type that it does
not recognize. Subpacket types 100 through 110 are reserved for
private or experimental use.
5.12.1. The Image Attribute Subpacket
The image attribute subpacket is used to encode an image, presumably
(but not required to be) that of the key owner.
The image attribute subpacket begins with an image header. The first
two octets of the image header contain the length of the image
header. Note that unlike other multi-octet numerical values in this
document, due to an historical accident this value is encoded as a
little-endian number. The image header length is followed by a
single octet for the image header version. The only currently
defined version of the image header is 1, which is a 16 octet image
header. The first three octets of a version 1 image header are thus
0x10 0x00 0x01.
The fourth octet of a version 1 image header designates the encoding
format of the image. The only currently defined encoding format is
the value 1 to indicate JPEG. Image format types 100 through 110 are
reserved for private or experimental use. The rest of the version 1
image header is made up of 12 reserved octets, all of which MUST be
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set to 0.
The rest of the image subpacket contains the image itself. As the
only currently defined image type is JPEG, the image is encoded in
the JPEG File Interchange Format (JFIF), a standard file format for
JPEG images. [JFIF]
An implementation MAY try and determine the type of an image by
examination of the image data if it is unable to handle a particular
version of the image header or if a specified encoding format value
is not recognized.
5.13. Sym. Encrypted Integrity Protected Data Packet (Tag 18)
The Symmetrically Encrypted Integrity Protected Data Packet is a
variant of the Symmetrically Encrypted Data Packet. It is a new
feature created for OpenPGP that addresses the problem of detecting
a modification to encrypted data. It is used in combination with a
Modification Detection Code Packet.
There is a corresponding feature in the features signature subpacket
that denotes that an implementation can properly use this packet
type. An implementation MUST support decrypting these packets and
SHOULD prefer generating them to the older Symmetrically Encrypted
Data Packet when possible. Since this data packet protects against
modification attacks, this standard encourages its proliferation.
While blanket adoption of this data packet would create
interoperability problems, rapid adoption is nevertheless important.
An implementation SHOULD specifically denote support for this
packet, but it MAY infer it from other mechanisms.
For example, an implementation might infer from the use of a cipher
such as AES or Twofish that a user supports this feature. It might
place in the unhashed portion of another user's key signature a
features subpacket. It might also present a user with an opportunity
to regenerate their own self-signature with a features subpacket.
This packet contains data encrypted with a symmetric-key algorithm
and protected against modification by the SHA-1 hash algorithm. When
it has been decrypted, it will typically contain other packets
(often a literal data packet or compressed data packet). The last
decrypted packet in this packet's payload MUST be a Modification
Detection Code packet.
The body of this packet consists of:
- A one-octet version number. The only currently defined value is
1.
- Encrypted data, the output of the selected symmetric-key cipher
operating in Cipher Feedback mode with shift amount equal to the
block size of the cipher (CFB-n where n is the block size).
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The symmetric cipher used MUST be specified in a Public-Key or
Symmetric-Key Encrypted Session Key packet that precedes the
Symmetrically Encrypted Data Packet. In either case, the cipher
algorithm octet is prefixed to the session key before it is
encrypted.
The data is encrypted in CFB mode, with a CFB shift size equal to
the cipher's block size. The Initial Vector (IV) is specified as all
zeros. Instead of using an IV, OpenPGP prefixes an octet string to
the data before it is encrypted. The length of the octet string
equals the block size of the cipher in octets, plus two. The first
octets in the group, of length equal to the block size of the
cipher, are random; the last two octets are each copies of their 2nd
preceding octet. For example, with a cipher whose block size is 128
bits or 16 octets, the prefix data will contain 16 random octets,
then two more octets, which are copies of the 15th and 16th octets,
respectively. Unlike the Symmetrically Encrypted Data Packet, no
special CFB resynchronization is done after encrypting this prefix
data. See OpenPGP CFB Mode below for more details.
The repetition of 16 bits in the random data prefixed to the message
allows the receiver to immediately check whether the session key is
incorrect.
The plaintext of the data to be encrypted is passed through the
SHA-1 hash function, and the result of the hash is appended to the
plaintext in a Modification Detection Code packet. The input to the
hash function includes the prefix data described above; it includes
all of the plaintext, and then also includes two octets of values
0xD3, 0x14. These represent the encoding of a Modification Detection
Code packet tag and length field of 20 octets.
The resulting hash value is stored in a Modification Detection Code
packet which MUST use the two octet encoding just given to represent
its tag and length field. The body of the MDC packet is the 20 octet
output of the SHA-1 hash.
The Modification Detection Code packet is appended to the plaintext
and encrypted along with the plaintext using the same CFB context.
During decryption, the plaintext data should be hashed with SHA-1,
including the prefix data as well as the packet tag and length field
of the Modification Detection Code packet. The body of the MDC
packet, upon decryption, is compared with the result of the SHA-1
hash.
Any failure of the MDC indicates that the message has been modified
and MUST be treated as a security problem. Failures include a
difference in the hash values, but also the absence of an MDC
packet, or an MDC packet in any position other than the end of the
plaintext. Any failure SHOULD be reported to the user.
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Note: future designs of new versions of this packet should consider
rollback attacks since it will be possible for an attacker to change
the version back to 1.
5.14. Modification Detection Code Packet (Tag 19)
The Modification Detection Code packet contains a SHA-1 hash of
plaintext data which is used to detect message modification. It is
only used with a Symmetrically Encrypted Integrity Protected Data
packet. The Modification Detection Code packet MUST be the last
packet in the plaintext data which is encrypted in the Symmetrically
Encrypted Integrity Protected Data packet, and MUST appear in no
other place.
A Modification Detection Code packet MUST have a length of 20
octets.
The body of this packet consists of:
- A 20-octet SHA-1 hash of the preceding plaintext data of the
Symmetrically Encrypted Integrity Protected Data packet,
including prefix data, the tag octet, and length octet of the
Modification Detection Code packet.
Note that the Modification Detection Code packet MUST always use a
new-format encoding of the packet tag, and a one-octet encoding of
the packet length. The reason for this is that the hashing rules for
modification detection include a one-octet tag and one-octet length
in the data hash. While this is a bit restrictive, it reduces
complexity.
6. Radix-64 Conversions
As stated in the introduction, OpenPGP's underlying native
representation for objects is a stream of arbitrary octets, and some
systems desire these objects to be immune to damage caused by
character set translation, data conversions, etc.
In principle, any printable encoding scheme that met the
requirements of the unsafe channel would suffice, since it would not
change the underlying binary bit streams of the native OpenPGP data
structures. The OpenPGP standard specifies one such printable
encoding scheme to ensure interoperability.
OpenPGP's Radix-64 encoding is composed of two parts: a base64
encoding of the binary data, and a checksum. The base64 encoding is
identical to the MIME base64 content-transfer-encoding [RFC2045].
The checksum is a 24-bit CRC converted to four characters of
radix-64 encoding by the same MIME base64 transformation, preceded
by an equals sign (=). The CRC is computed by using the generator
0x864CFB and an initialization of 0xB704CE. The accumulation is done
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on the data before it is converted to radix-64, rather than on the
converted data. A sample implementation of this algorithm is in the
next section.
The checksum with its leading equal sign MAY appear on the first
line after the Base64 encoded data.
Rationale for CRC-24: The size of 24 bits fits evenly into printable
base64. The nonzero initialization can detect more errors than a
zero initialization.
6.1. An Implementation of the CRC-24 in "C"
#define CRC24_INIT 0xb704ceL
#define CRC24_POLY 0x1864cfbL
typedef long crc24;
crc24 crc_octets(unsigned char *octets, size_t len)
{
crc24 crc = CRC24_INIT;
int i;
while (len--) {
crc ^= (*octets++) << 16;
for (i = 0; i < 8; i++) {
crc <<= 1;
if (crc & 0x1000000)
crc ^= CRC24_POLY;
}
}
return crc & 0xffffffL;
}
6.2. Forming ASCII Armor
When OpenPGP encodes data into ASCII Armor, it puts specific headers
around the Radix-64 encoded data, so OpenPGP can reconstruct the
data later. An OpenPGP implementation MAY use ASCII armor to protect
raw binary data. OpenPGP informs the user what kind of data is
encoded in the ASCII armor through the use of the headers.
Concatenating the following data creates ASCII Armor:
- An Armor Header Line, appropriate for the type of data
- Armor Headers
- A blank (zero-length, or containing only whitespace) line
- The ASCII-Armored data
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- An Armor Checksum
- The Armor Tail, which depends on the Armor Header Line.
An Armor Header Line consists of the appropriate header line text
surrounded by five (5) dashes ('-', 0x2D) on either side of the
header line text. The header line text is chosen based upon the type
of data that is being encoded in Armor, and how it is being encoded.
Header line texts include the following strings:
BEGIN PGP MESSAGE
Used for signed, encrypted, or compressed files.
BEGIN PGP PUBLIC KEY BLOCK
Used for armoring public keys
BEGIN PGP PRIVATE KEY BLOCK
Used for armoring private keys
BEGIN PGP MESSAGE, PART X/Y
Used for multi-part messages, where the armor is split amongst Y
parts, and this is the Xth part out of Y.
BEGIN PGP MESSAGE, PART X
Used for multi-part messages, where this is the Xth part of an
unspecified number of parts. Requires the MESSAGE-ID Armor
Header to be used.
BEGIN PGP SIGNATURE
Used for detached signatures, OpenPGP/MIME signatures, and
cleartext signatures. Note that PGP 2.x uses BEGIN PGP MESSAGE
for detached signatures.
Note that all these Armor Header Lines are to consist of a complete
line. That is to say, there is always a line ending preceding the
starting five dashes, and following the ending five dashes. The
header lines, therefore, MUST start at the beginning of a line, and
MUST NOT have text other than whitespace following them on the same
line. These line endings are considered a part of the Armor Header
Line for the purposes of determining the content they delimit. This
is particularly important when computing a cleartext signature (see
below).
The Armor Headers are pairs of strings that can give the user or the
receiving OpenPGP implementation some information about how to
decode or use the message. The Armor Headers are a part of the
armor, not a part of the message, and hence are not protected by any
signatures applied to the message.
The format of an Armor Header is that of a key-value pair. A colon
(':' 0x38) and a single space (0x20) separate the key and value.
OpenPGP should consider improperly formatted Armor Headers to be
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corruption of the ASCII Armor. Unknown keys should be reported to
the user, but OpenPGP should continue to process the message.
Note that some transport methods are sensitive to line length. While
there is a limit of 76 characters for the Radix-64 data (section6.3), there is no limit to the length of Armor Headers. Care should
be taken that the Armor Headers are short enough to survive
transport. One way to do this is to repeat an Armor Header key
multiple times with different values for each so that no one line is
overly long.
Currently defined Armor Header Keys are:
- "Version", that states the OpenPGP implementation and version
used to encode the message.
- "Comment", a user-defined comment. OpenPGP defines all text to
be in UTF-8. A comment may be any UTF-8 string. However, the
whole point of armoring is to provide seven-bit-clean data.
Consequently, if a comment has characters that are outside the
US-ASCII range of UTF, they may very well not survive transport.
- "MessageID", a 32-character string of printable characters. The
string must be the same for all parts of a multi-part message
that uses the "PART X" Armor Header. MessageID strings should be
unique enough that the recipient of the mail can associate all
the parts of a message with each other. A good checksum or
cryptographic hash function is sufficient.
The MessageID SHOULD NOT appear unless it is in a multi-part
message. If it appears at all, it MUST be computed from the
finished (encrypted, signed, etc.) message in a deterministic
fashion, rather than contain a purely random value. This is to
allow the legitimate recipient to determine that the MessageID
cannot serve as a covert means of leaking cryptographic key
information.
- "Hash", a comma-separated list of hash algorithms used in this
message. This is used only in cleartext signed messages.
- "Charset", a description of the character set that the plaintext
is in. Please note that OpenPGP defines text to be in UTF-8. An
implementation will get best results by translating into and out
of UTF-8. However, there are many instances where this is easier
said than done. Also, there are communities of users who have no
need for UTF-8 because they are all happy with a character set
like ISO Latin-5 or a Japanese character set. In such instances,
an implementation MAY override the UTF-8 default by using this
header key. An implementation MAY implement this key and any
translations it cares to; an implementation MAY ignore it and
assume all text is UTF-8.
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1. The last data group has 24 bits (3 octets). No special
processing is needed.
2. The last data group has 16 bits (2 octets). The first two 6-bit
groups are processed as above. The third (incomplete) data group
has two zero-value bits added to it, and is processed as above.
A pad character (=) is added to the output.
3. The last data group has 8 bits (1 octet). The first 6-bit group
is processed as above. The second (incomplete) data group has
four zero-value bits added to it, and is processed as above. Two
pad characters (=) are added to the output.
6.4. Decoding Radix-64
In Radix-64 data, characters other than those in the table, line
breaks, and other white space probably indicate a transmission
error, about which a warning message or even a message rejection
might be appropriate under some circumstances. Decoding software
must ignore all white space.
Because it is used only for padding at the end of the data, the
occurrence of any "=" characters may be taken as evidence that the
end of the data has been reached (without truncation in transit). No
such assurance is possible, however, when the number of octets
transmitted was a multiple of three and no "=" characters are
present.
6.5. Examples of Radix-64
Input data: 0x14fb9c03d97e
Hex: 1 4 f b 9 c | 0 3 d 9 7 e
8-bit: 00010100 11111011 10011100 | 00000011 11011001
11111110
6-bit: 000101 001111 101110 011100 | 000000 111101 100111
111110
Decimal: 5 15 46 28 0 61 37 62
Output: F P u c A 9 l +
Input data: 0x14fb9c03d9
Hex: 1 4 f b 9 c | 0 3 d 9
8-bit: 00010100 11111011 10011100 | 00000011 11011001
pad with 00
6-bit: 000101 001111 101110 011100 | 000000 111101 100100
Decimal: 5 15 46 28 0 61 36
pad with =
Output: F P u c A 9 k =
Input data: 0x14fb9c03
Hex: 1 4 f b 9 c | 0 3
8-bit: 00010100 11111011 10011100 | 00000011
pad with 0000
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6-bit: 000101 001111 101110 011100 | 000000 110000
Decimal: 5 15 46 28 0 48
pad with = =
Output: F P u c A w = =
6.6. Example of an ASCII Armored Message
-----BEGIN PGP MESSAGE-----
Version: OpenPrivacy 0.99
yDgBO22WxBHv7O8X7O/jygAEzol56iUKiXmV+XmpCtmpqQUKiQrFqclFqUDBovzS
vBSFjNSiVHsuAA==
=njUN
-----END PGP MESSAGE-----
Note that this example has extra indenting; an actual armored
message would have no leading whitespace.
7. Cleartext signature framework
It is desirable to sign a textual octet stream without ASCII
armoring the stream itself, so the signed text is still readable
without special software. In order to bind a signature to such a
cleartext, this framework is used. (Note that this framework is not
intended to be reversible. RFC 3156 defines another way to sign
cleartext messages for environments that support MIME.)
The cleartext signed message consists of:
- The cleartext header '-----BEGIN PGP SIGNED MESSAGE-----' on a
single line,
- One or more "Hash" Armor Headers,
- Exactly one empty line not included into the message digest,
- The dash-escaped cleartext that is included into the message
digest,
- The ASCII armored signature(s) including the '-----BEGIN PGP
SIGNATURE-----' Armor Header and Armor Tail Lines.
If the "Hash" armor header is given, the specified message digest
algorithm(s) are used for the signature. If there are no such
headers, MD5 is used. If MD5 is the only hash used, then an
implementation MAY omit this header for improved V2.x compatibility.
If more than one message digest is used in the signature, the "Hash"
armor header contains a comma-delimited list of used message
digests.
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Current message digest names are described below with the algorithm
IDs.
An implementation SHOULD add a line break after the cleartext, but
MAY omit it if the cleartext ends with a line break. This is for
visual clarity.
7.1. Dash-Escaped Text
The cleartext content of the message must also be dash-escaped.
Dash escaped cleartext is the ordinary cleartext where every line
starting with a dash '-' (0x2D) is prefixed by the sequence dash '-'
(0x2D) and space ' ' (0x20). This prevents the parser from
recognizing armor headers of the cleartext itself. An implementation
MAY dash escape any line, SHOULD dash escape lines commencing "From"
followed by a space, and MUST dash escape any line commencing in a
dash. The message digest is computed using the cleartext itself, not
the dash escaped form.
As with binary signatures on text documents, a cleartext signature
is calculated on the text using canonical <CR><LF> line endings. The
line ending (i.e. the <CR><LF>) before the '-----BEGIN PGP
SIGNATURE-----' line that terminates the signed text is not
considered part of the signed text.
When reversing dash-escaping, an implementation MUST strip the
string "- " if it occurs at the beginning of a line, and SHOULD warn
on "-" and any character other than a space at the beginning of a
line.
Also, any trailing whitespace -- spaces (0x20) and tabs (0x09) -- at
the end of any line is removed when the cleartext signature is
generated.
8. Regular Expressions
A regular expression is zero or more branches, separated by '|'. It
matches anything that matches one of the branches.
A branch is zero or more pieces, concatenated. It matches a match
for the first, followed by a match for the second, etc.
A piece is an atom possibly followed by '*', '+', or '?'. An atom
followed by '*' matches a sequence of 0 or more matches of the atom.
An atom followed by '+' matches a sequence of 1 or more matches of
the atom. An atom followed by '?' matches a match of the atom, or
the null string.
An atom is a regular expression in parentheses (matching a match for
the regular expression), a range (see below), '.' (matching any
single character), '^' (matching the null string at the beginning of
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the input string), '$' (matching the null string at the end of the
input string), a '\' followed by a single character (matching that
character), or a single character with no other significance
(matching that character).
A range is a sequence of characters enclosed in '[]'. It normally
matches any single character from the sequence. If the sequence
begins with '^', it matches any single character not from the rest
of the sequence. If two characters in the sequence are separated by
'-', this is shorthand for the full list of ASCII characters between
them (e.g. '[0-9]' matches any decimal digit). To include a literal
']' in the sequence, make it the first character (following a
possible '^'). To include a literal '-', make it the first or last
character.
9. Constants
This section describes the constants used in OpenPGP.
Note that these tables are not exhaustive lists; an implementation
MAY implement an algorithm not on these lists, so long as the
algorithm number(s) are chosen from the private or experimental
algorithm range.
See the section "Notes on Algorithms" below for more discussion of
the algorithms.
9.1. Public Key Algorithms
ID Algorithm
-- ---------
1 - RSA (Encrypt or Sign) [HAC]
2 - RSA Encrypt-Only
3 - RSA Sign-Only
16 - Elgamal (Encrypt-Only), see [ELGAMAL] [HAC]
17 - DSA (Digital Signature Algorithm) [FIPS186] [HAC]
18 - Reserved for Elliptic Curve
19 - Reserved for ECDSA
20 - Reserved (formerly Elgamal Encrypt or Sign)
21 - Reserved for Diffie-Hellman (X9.42,
as defined for IETF-S/MIME)
100 to 110 - Private/Experimental algorithm.
Implementations MUST implement DSA for signatures, and Elgamal for
encryption. Implementations SHOULD implement RSA keys.
Implementations MAY implement any other algorithm.
9.2. Symmetric Key Algorithms
ID Algorithm
-- ---------
0 - Plaintext or unencrypted data
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Implementations MUST implement SHA-1. Implementations MAY implement
other algorithms. MD5 is deprecated.
10. Packet Composition
OpenPGP packets are assembled into sequences in order to create
messages and to transfer keys. Not all possible packet sequences are
meaningful and correct. This section describes the rules for how
packets should be placed into sequences.
10.1. Transferable Public Keys
OpenPGP users may transfer public keys. The essential elements of a
transferable public key are:
- One Public Key packet
- Zero or more revocation signatures
- One or more User ID packets
- After each User ID packet, zero or more signature packets
(certifications)
- Zero or more User Attribute packets
- After each User Attribute packet, zero or more signature packets
(certifications)
- Zero or more Subkey packets
- After each Subkey packet, one signature packet, plus optionally
a revocation.
The Public Key packet occurs first. Each of the following User ID
packets provides the identity of the owner of this public key. If
there are multiple User ID packets, this corresponds to multiple
means of identifying the same unique individual user; for example, a
user may have more than one email address, and construct a User ID
for each one.
Immediately following each User ID packet, there are zero or more
signature packets. Each signature packet is calculated on the
immediately preceding User ID packet and the initial Public Key
packet. The signature serves to certify the corresponding public key
and User ID. In effect, the signer is testifying to his or her
belief that this public key belongs to the user identified by this
User ID.
Within the same section as the User ID packets, there are zero or
more User Attribute packets. Like the User ID packets, a User
Attribute packet is followed by zero or more signature packets
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calculated on the immediately preceding User Attribute packet and
the initial Public Key packet.
User Attribute packets and User ID packets may be freely intermixed
in this section, so long as the signatures that follow them are
maintained on the proper User Attribute or User ID packet.
After the User ID or Attribute packets there may be zero or more
Subkey packets. In general, subkeys are provided in cases where the
top-level public key is a signature-only key. However, any V4 key
may have subkeys, and the subkeys may be encryption-only keys,
signature-only keys, or general-purpose keys. V3 keys MUST NOT have
subkeys.
Each Subkey packet MUST be followed by one Signature packet, which
should be a subkey binding signature issued by the top level key.
For subkeys that can issue signatures, the subkey binding signature
MUST contain an embedded signature subpacket with a primary key
binding signature (0x19) issued by the subkey on the top level key.
Subkey and Key packets may each be followed by a revocation
Signature packet to indicate that the key is revoked. Revocation
signatures are only accepted if they are issued by the key itself,
or by a key that is authorized to issue revocations via a revocation
key subpacket in a self-signature by the top level key.
Transferable public key packet sequences may be concatenated to
allow transferring multiple public keys in one operation.
10.2. Transferable Secret Keys
OpenPGP users may transfer secret keys. The format of a transferable
secret key is the same as a transferable public key except that
secret key and secret subkey packets are used instead of the public
key and public subkey packets. Implementations SHOULD include
self-signatures on any user IDs and subkeys, as this allows for a
complete public key to be automatically extracted from the
transferable secret key. Implementations MAY choose to omit the
self-signatures, especially if a transferable public key accompanies
the transferable secret key.
10.3. OpenPGP Messages
An OpenPGP message is a packet or sequence of packets that
corresponds to the following grammatical rules (comma represents
sequential composition, and vertical bar separates alternatives):
OpenPGP Message :- Encrypted Message | Signed Message |
Compressed Message | Literal Message.
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The format of an OpenPGP V4 key that uses multiple public keys is
similar except that the other keys are added to the end as "subkeys"
of the primary key.
Primary-Key
[Revocation Self Signature]
[Direct Key Signature...]
User ID [Signature ...]
[User ID [Signature ...] ...]
[User Attribute [Signature ...] ...]
[[Subkey [Binding-Signature-Revocation]
Primary-Key-Binding-Signature] ...]
A subkey always has a single signature after it that is issued using
the primary key to tie the two keys together. This binding signature
may be in either V3 or V4 format, but SHOULD be V4. Subkeys that can
issue signatures MUST have a V4 binding signature due to the
REQUIRED embedded primary key binding signature.
In the above diagram, if the binding signature of a subkey has been
revoked, the revoked key may be removed, leaving only one key.
In a V4 key, the primary key MUST be a key capable of certification.
The subkeys may be keys of any other type. There may be other
constructions of V4 keys, too. For example, there may be a
single-key RSA key in V4 format, a DSA primary key with an RSA
encryption key, or RSA primary key with an Elgamal subkey, etc.
It is also possible to have a signature-only subkey. This permits a
primary key that collects certifications (key signatures) but is
used only used for certifying subkeys that are used for encryption
and signatures.
11.2. Key IDs and Fingerprints
For a V3 key, the eight-octet key ID consists of the low 64 bits of
the public modulus of the RSA key.
The fingerprint of a V3 key is formed by hashing the body (but not
the two-octet length) of the MPIs that form the key material (public
modulus n, followed by exponent e) with MD5. Note that both V3 keys
and MD5 are deprecated.
A V4 fingerprint is the 160-bit SHA-1 hash of the octet 0x99,
followed by the two-octet packet length, followed by the entire
Public Key packet starting with the version field. The key ID is the
low order 64 bits of the fingerprint. Here are the fields of the
hash material, with the example of a DSA key:
a.1) 0x99 (1 octet)
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a.2) high order length octet of (b)-(f) (1 octet)
a.3) low order length octet of (b)-(f) (1 octet)
b) version number = 4 (1 octet);
c) time stamp of key creation (4 octets);
d) algorithm (1 octet): 17 = DSA (example);
e) Algorithm specific fields.
Algorithm Specific Fields for DSA keys (example):
e.1) MPI of DSA prime p;
e.2) MPI of DSA group order q (q is a prime divisor of p-1);
e.3) MPI of DSA group generator g;
e.4) MPI of DSA public key value y (= g**x mod p where x is secret).
Note that it is possible for there to be collisions of key IDs --
two different keys with the same key ID. Note that there is a much
smaller, but still non-zero probability that two different keys have
the same fingerprint.
Also note that if V3 and V4 format keys share the same RSA key
material, they will have different key IDs as well as different
fingerprints.
Finally, the key ID and fingerprint of a subkey are calculated in
the same way as for a primary key, including the 0x99 as the first
octet (even though this is not a valid packet ID for a public
subkey).
12. Notes on Algorithms12.1. PKCS#1 Encoding In OpenPGP
This standard makes use of the PKCS#1 functions EME-PKCS1-v1_5 and
EMSA-PKCS1-v1_5. However, the calling conventions of these
functions has changed in the past. To avoid potential confusion and
interoperability problems, we are including local copies in this
document, adapted from those in PKCS#1 v2.1 [RFC3447]. RFC-3447
should be treated as the ultimate authority on PKCS#1 for OpenPGP.
Nonetheless, we believe that there is value in having a
self-contained document that avoids problems in the future with
needed changes in the conventions.
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EM = 0x00 || 0x01 || PS || 0x00 || T.
6. Output EM.
12.2. Symmetric Algorithm Preferences
The symmetric algorithm preference is an ordered list of algorithms
that the keyholder accepts. Since it is found on a self-signature,
it is possible that a keyholder may have multiple, different
preferences. For example, Alice may have TripleDES only specified
for "alice@work.com" but CAST5, Blowfish, and TripleDES specified
for "alice@home.org". Note that it is also possible for preferences
to be in a subkey's binding signature.
Since TripleDES is the MUST-implement algorithm, if it is not
explicitly in the list, it is tacitly at the end. However, it is
good form to place it there explicitly. Note also that if an
implementation does not implement the preference, then it is
implicitly a TripleDES-only implementation.
An implementation MUST NOT use a symmetric algorithm that is not in
the recipient's preference list. When encrypting to more than one
recipient, the implementation finds a suitable algorithm by taking
the intersection of the preferences of the recipients. Note that the
MUST-implement algorithm, TripleDES, ensures that the intersection
is not null. The implementation may use any mechanism to pick an
algorithm in the intersection.
If an implementation can decrypt a message that a keyholder doesn't
have in their preferences, the implementation SHOULD decrypt the
message anyway, but MUST warn the keyholder that the protocol has
been violated. For example, suppose that Alice, above, has software
that implements all algorithms in this specification. Nonetheless,
she prefers subsets for work or home. If she is sent a message
encrypted with IDEA, which is not in her preferences, the software
warns her that someone sent her an IDEA-encrypted message, but it
would ideally decrypt it anyway.
12.3. Other Algorithm Preferences
Other algorithm preferences work similarly to the symmetric
algorithm preference, in that they specify which algorithms the
keyholder accepts. There are two interesting cases that other
comments need to be made about, though, the compression preferences
and the hash preferences.
12.3.1. Compression Preferences
Compression has been an integral part of PGP since its first days.
OpenPGP and all previous versions of PGP have offered compression.
In this specification, the default is for messages to be compressed,
although an implementation is not required to do so. Consequently,
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the compression preference gives a way for a keyholder to request
that messages not be compressed, presumably because they are using a
minimal implementation that does not include compression.
Additionally, this gives a keyholder a way to state that it can
support alternate algorithms.
Like the algorithm preferences, an implementation MUST NOT use an
algorithm that is not in the preference vector. If the preferences
are not present, then they are assumed to be [ZIP(1),
UNCOMPRESSED(0)].
Additionally, an implementation MUST implement this preference to
the degree of recognizing when to send an uncompressed message. A
robust implementation would satisfy this requirement by looking at
the recipient's preference and acting accordingly. A minimal
implementation can satisfy this requirement by never generating a
compressed message, since all implementations can handle messages
that have not been compressed.
12.3.2. Hash Algorithm Preferences
Typically, the choice of a hash algorithm is something the signer
does, rather than the verifier, because a signer rarely knows who is
going to be verifying the signature. This preference, though, allows
a protocol based upon digital signatures ease in negotiation.
Thus, if Alice is authenticating herself to Bob with a signature, it
makes sense for her to use a hash algorithm that Bob's software
uses. This preference allows Bob to state in his key which
algorithms Alice may use.
Since SHA1 is the MUST-implement hash algorithm, if it is not
explicitly in the list, it is tacitly at the end. However, it is
good form to place it there explicitly.
12.4. Plaintext
Algorithm 0, "plaintext," may only be used to denote secret keys
that are stored in the clear. Implementations MUST NOT use plaintext
in Symmetrically Encrypted Data Packets; they must use Literal Data
Packets to encode unencrypted or literal data.
12.5. RSA
There are algorithm types for RSA-signature-only, and
RSA-encrypt-only keys. These types are deprecated. The "key flags"
subpacket in a signature is a much better way to express the same
idea, and generalizes it to all algorithms. An implementation SHOULD
NOT create such a key, but MAY interpret it.
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An implementation SHOULD NOT implement RSA keys of size less than
1024 bits.
12.6. DSA
An implementation SHOULD NOT implement DSA keys of size less than
1024 bits. It MUST NOT implement a DSA key with a q size of less
than 160 bits. DSA keys MUST also be a multiple of 64 bits, and the
q size MUST be a multiple of 8 bits. The Digital Signature Standard
(DSS) [FIPS186] specifies that DSA be used in one of the following
ways:
* 1024-bit key, 160-bit q, SHA-1, SHA-224, SHA-256, SHA-384 or
SHA-512 hash
* 2048-bit key, 224-bit q, SHA-224, SHA-256, SHA-384 or SHA-512
hash
* 2048-bit key, 256-bit q, SHA-256, SHA-384 or SHA-512 hash
* 3072-bit key, 256-bit q, SHA-256, SHA-384 or SHA-512 hash
The above key and q size pairs were chosen to best balance the
strength of the key with the strength of the hash. Implementations
SHOULD use one of the above key and q size pairs when generating DSA
keys. If DSS compliance is desired, one of the specified SHA hashes
must be used as well. [FIPS186] is the ultimate authority on DSS,
and should be consulted for all questions of DSS compliance.
Note that earlier versions of this standard only allowed a 160-bit q
with no truncation allowed, so earlier implementations may not be
able to handle signatures with a different q size or a truncated
hash.
12.7. Elgamal
An implementation SHOULD NOT implement Elgamal keys of size less
than 1024 bits.
12.8. Reserved Algorithm Numbers
A number of algorithm IDs have been reserved for algorithms that
would be useful to use in an OpenPGP implementation, yet there are
issues that prevent an implementer from actually implementing the
algorithm. These are marked in the Public Algorithms section as
"(reserved for)".
The reserved public key algorithms, Elliptic Curve (18), ECDSA (19),
and X9.42 (21) do not have the necessary parameters, parameter
order, or semantics defined.
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Previous versions of OpenPGP permitted Elgamal [ELGAMAL] signatures
with a public key identifier of 20. These are no longer permitted.
An implementation MUST NOT generate such keys. An implementation
MUST NOT generate Elgamal signatures.
12.9. OpenPGP CFB mode
OpenPGP does symmetric encryption using a variant of Cipher Feedback
Mode (CFB mode). This section describes the procedure it uses in
detail. This mode is what is used for Symmetrically Encrypted Data
Packets; the mechanism used for encrypting secret key material is
similar, but described in those sections above.
In the description below, the value BS is the block size in octets
of the cipher. Most ciphers have a block size of 8 octets. The AES
and Twofish have a block size of 16 octets. Also note that the
description below assumes that the IV and CFB arrays start with an
index of 1 (unlike the C language, which assumes arrays start with a
zero index).
OpenPGP CFB mode uses an initialization vector (IV) of all zeros,
and prefixes the plaintext with BS+2 octets of random data, such
that octets BS+1 and BS+2 match octets BS-1 and BS. It does a CFB
resynchronization after encrypting those BS+2 octets.
Thus, for an algorithm that has a block size of 8 octets (64 bits),
the IV is 10 octets long and octets 7 and 8 of the IV are the same
as octets 9 and 10. For an algorithm with a block size of 16 octets
(128 bits), the IV is 18 octets long, and octets 17 and 18 replicate
octets 15 and 16. Those extra two octets are an easy check for a
correct key.
Step by step, here is the procedure:
1. The feedback register (FR) is set to the IV, which is all zeros.
2. FR is encrypted to produce FRE (FR Encrypted). This is the
encryption of an all-zero value.
3. FRE is xored with the first BS octets of random data prefixed to
the plaintext to produce C[1] through C[BS], the first BS octets
of ciphertext.
4. FR is loaded with C[1] through C[BS].
5. FR is encrypted to produce FRE, the encryption of the first BS
octets of ciphertext.
6. The left two octets of FRE get xored with the next two octets of
data that were prefixed to the plaintext. This produces C[BS+1]
and C[BS+2], the next two octets of ciphertext.
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7. (The resynchronization step) FR is loaded with C[3] through
C[BS+2].
8. FR is encrypted to produce FRE.
9. FRE is xored with the first BS octets of the given plaintext,
now that we have finished encrypting the BS+2 octets of prefixed
data. This produces C[BS+3] through C[BS+(BS+2)], the next BS
octets of ciphertext.
10. FR is loaded with C[BS+3] to C[BS + (BS+2)] (which is C11-C18
for an 8-octet block).
11. FR is encrypted to produce FRE.
12. FRE is xored with the next BS octets of plaintext, to produce
the next BS octets of ciphertext. These are loaded into FR and
the process is repeated until the plaintext is used up.
13. Security Considerations
* As with any technology involving cryptography, you should check
the current literature to determine if any algorithms used here
have been found to be vulnerable to attack.
* This specification uses Public Key Cryptography technologies. It
is assumed that the private key portion of a public-private key
pair is controlled and secured by the proper party or parties.
* Certain operations in this specification involve the use of
random numbers. An appropriate entropy source should be used to
generate these numbers. See RFC 1750.
* The MD5 hash algorithm has been found to have weaknesses, with
collisions found in a number of cases. MD5 is deprecated for use
in OpenPGP. Implementations MUST NOT generate new signatures
using MD5 as a hash function. They MAY continue to consider old
signatures that used MD5 as valid.
* SHA-224 and SHA-384 require the same work as SHA-256 and SHA-512
respectively. In general, there are few reasons to use them
outside of DSS compatibility. You need a situation where one
needs more security than smaller hashes, but does not want to
have the full 256-bit or 512-bit data length.
* Many security protocol designers think that it is a bad idea to
use a single key for both privacy (encryption) and integrity
(signatures). In fact, this was one of the motivating forces
behind the V4 key format with separate signature and encryption
keys. If you as an implementer promote dual-use keys, you should
at least be aware of this controversy.
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* The DSA algorithm will work with any hash, but is sensitive to
the quality of the hash algorithm. Verifiers should be aware
that even if the signer used a strong hash, an attacker could
have modified the signature to use a weak one. Only signatures
using acceptably strong hash algorithms should be accepted as
valid.
* As OpenPGP combines many different asymmetric, symmetric, and
hash algorithms, each with different measures of strength, care
should be taken that the weakest element of an OpenPGP message
is still sufficiently strong for the purpose at hand. While
consensus about the the strength of a given algorithm may
evolve, NIST Special Publication 800-57 [SP800-57] recommends
the following list of equivalent strengths:
Asymmetric | Hash | Symmetric
key size | size | key size
------------+--------+-----------
1024 160 80
2048 224 112
3072 256 128
7680 384 192
15360 512 256
* There is a somewhat-related potential security problem in
signatures. If an attacker can find a message that hashes to the
same hash with a different algorithm, a bogus signature
structure can be constructed that evaluates correctly.
For example, suppose Alice DSA signs message M using hash
algorithm H. Suppose that Mallet finds a message M' that has the
same hash value as M with H'. Mallet can then construct a
signature block that verifies as Alice's signature of M' with
H'. However, this would also constitute a weakness in either H
or H' or both. Should this ever occur, a revision will have to
be made to this document to revise the allowed hash algorithms.
* If you are building an authentication system, the recipient may
specify a preferred signing algorithm. However, the signer would
be foolish to use a weak algorithm simply because the recipient
requests it.
* Some of the encryption algorithms mentioned in this document
have been analyzed less than others. For example, although CAST5
is presently considered strong, it has been analyzed less than
TripleDES. Other algorithms may have other controversies
surrounding them.
* In late summer 2002, Jallad, Katz, and Schneier published an
interesting attack on the OpenPGP protocol and some of its
implementations [JKS02]. In this attack, the attacker modifies a
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message and sends it to a user who then returns the erroneously
decrypted message to the attacker. The attacker is thus using
the user as a random oracle, and can often decrypt the message.
Compressing data can ameliorate this attack. The incorrectly
decrypted data nearly always decompresses in ways that defeats
the attack. However, this is not a rigorous fix, and leaves open
some small vulnerabilities. For example, if an implementation
does not compress a message before encryption (perhaps because
it knows it was already compressed), then that message is
vulnerable. Because of this happenstance -- that modification
attacks can be thwarted by decompression errors, an
implementation SHOULD treat a decompression error as a security
problem, not merely a data problem.
This attack can be defeated by the use of Modification
Detection, provided that the implementation does not let the
user naively return the data to the attacker. An implementation
MUST treat an MDC failure as a security problem, not merely a
data problem.
In either case, the implementation MAY allow the user access to
the erroneous data, but MUST warn the user as to potential
security problems should that data be returned to the sender.
While this attack is somewhat obscure, requiring a special set
of circumstances to create it, it is nonetheless quite serious
as it permits someone to trick a user to decrypt a message.
Consequently, it is important that:
1. Implementers treat MDC errors and decompression failures as
security problems.
2. Implementers implement Modification Detection with all due
speed and encourage its spread.
3. Users migrate to implementations that support Modification
Detection with all due speed.
* PKCS#1 has been found to be vulnerable to attacks in which a
system that reports errors in padding differently from errors in
decryption becomes a random oracle that can leak the private key
in mere millions of queries. Implementations must be aware of
this attack and prevent it from happening. The simplest solution
is report a single error code for all variants of decryption
errors so as not to leak information to an attacker.
* Some technologies mentioned here may be subject to government
control in some countries.
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* In winter 2005, Serge Mister and Robert Zuccherato from Entrust
released a paper describing a way that the "quick check" in
OpenPGP CFB mode can be used with a random oracle to decrypt two
octets of every cipher block [MZ05]. They recommend as
prevention not using the quick check at all.
Many implementers have taken this advice to heart for any data
that is symmetrically encrypted and for which the session key is
public-key encrypted. In this case, the quick check is not
needed as the public key encryption of the session key should
guarantee that it is the right session key. In other cases, the
implementation should use the quick check with care.
On the one hand, there is a danger to using it if there is a
random oracle that can leak information to an attacker. In
plainer language, there is a danger to using the quick check if
timing information about the check can be exposed to an
attacker, particularly via an automated service that allows
rapidly repeated queries.
On the other hand, it is inconvenient to the user to be informed
that they typed in the wrong passphrase only after a petabyte of
data is decrypted. There are many cases in cryptographic
engineering where the implementer must use care and wisdom, and
this is one.
14. Implementation Nits
This section is a collection of comments to help an implementer,
particularly with an eye to backward compatibility. Previous
implementations of PGP are not OpenPGP-compliant. Often the
differences are small, but small differences are frequently more
vexing than large differences. Thus, this is a non-comprehensive
list of potential problems and gotchas for a developer who is trying
to be backward-compatible.
* The IDEA algorithm is patented, and yet it is required for PGP
2.x interoperability. It is also the defacto preferred algorithm
for a V3 key with a V3 self-signature (or no self-signature).
* When exporting a private key, PGP 2.x generates the header
"BEGIN PGP SECRET KEY BLOCK" instead of "BEGIN PGP PRIVATE KEY
BLOCK". All previous versions ignore the implied data type, and
look directly at the packet data type.
* PGP 2.0 through 2.5 generated V2 Public Key Packets. These are
identical to the deprecated V3 keys except for the version
number. An implementation MUST NOT generate them and may accept
or reject them as it sees fit. Some older PGP versions generated
V2 PKESK packets (Tag 1) as well. An implementation may accept
or reject V2 PKESK packets as it sees fit, and MUST NOT generate
them.
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* PGP 2.6.x will not accept key-material packets with versions
greater than 3.
* There are many ways possible for two keys to have the same key
material, but different fingerprints (and thus key IDs). Perhaps
the most interesting is an RSA key that has been "upgraded" to
V4 format, but since a V4 fingerprint is constructed by hashing
the key creation time along with other things, two V4 keys
created at different times, yet with the same key material will
have different fingerprints.
* If an implementation is using zlib to interoperate with PGP 2.x,
then the "windowBits" parameter should be set to -13.
* The 0x19 back signatures were not required for signing subkeys
until relatively recently. Consquently, there may be keys in the
wild that do not have these back signatures. Implementing
software may handle these keys as it sees fit.
15. Authors' Addresses
The working group can be contacted via the current chair:
Derek Atkins
IHTFP Consulting, Inc.
6 Farragut Ave
Somerville, MA 02144 USA
Email: derek@ihtfp.com
Tel: +1 617 623 3745
The principal authors of this draft are:
Jon Callas
Email: jon@callas.org
Lutz Donnerhacke
IKS GmbH
Wildenbruchstr. 15
07745 Jena, Germany
EMail: lutz@iks-jena.de
Hal Finney
Email: hal@finney.org
David Shaw
Email: dshaw@jabberwocky.com
Rodney Thayer
Email: rodney@tillerman.to
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